The formulation de ces, where de is partitive, is okay and roughly means "some of these/those".
The formulation can indeed sound strange to English ears, since matching the words suggests that something is lacking:
They / have / some / of / those / weapons
Ils / ont / ____ / de / ces / armes
If you were to insert quelques-unes, then this de would be a preposition, and that construction is unambiguously unmarked (e.g. in phrases like l'un de ces trois).
We do have to face it as a partitive here, but we don't have to translate it word for word. After all, we're used to the difficulty of translating partitive de in other phrases. We know that without further context, du regret, de la farine, and des biscuits are best translated using "some" or no determiner at all.
Here, de ces is best translated "some of those":
J'ai mangé de ces biscuits de Noël que tu m'as donnés.
A straightforward translation of the Astérix quote would thus be:
These Gauls have some of those secret weapons that should have been banned by the International Commission of the Helvetii!
However, if pressed we couldn't say "some" is an inherent part of the meaning of de ces, because I didn't see it filling a subject role in any examples (whereas the English "some of those" can).
Yet it might not be surprising if we translate de ces as "of those". For example, in archaic English this parallel structure sounds rather poetic:
And the woman said unto the serpent, We may eat of the fruit of the trees of the garden: But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die.
(Genesis 3:2-3, King James Version)
Is it colloquial?
Some people in this thread feel strongly that it is, and others that it isn't. After examining a few dozen uses in books over the last two centuries found through an Ngram search for just one formulation — avoir de ces — I'm obliged to say that it isn't inherently colloquial. It appears in many normal and formal contexts. That said, its use has, on the whole, dropped — at least in books, which might signal the rise of the colloquial usage compared to older authors' usage:
When reading through the examples, I weeded out tricky cases that actually involve prepositions (unfortunately didn't retain the links for these ones):
Il faut avoir de ces spectacles une opinion élevée
La part qu'ils doivent avoir de ces retours
Il voulait ce qu'il pouvait avoir de ces profits
I also noted that many cases are in negative structures, perhaps not applicable because de seems to appear in such structures even when it would be absent in the positive:
Ceux qui ne pourront pas avoir de ces terreins (link)
Il est triste de n'avoir de ces bonnes fortunes-là qu'une fois par an (link)
i.e., the de here may not be contributing a partitive meaning, as in your own question elsewhere:
Je ne voulais pas de son amour.
But even omitting the former type and taking the latter with a grain of salt, there are tons of unambiguous examples:
Pour avoir de ces vues, il faut avoir ... (link)
Il peut y avoir de ces pierres en forme de concombre (link)
Il reste à voir combien il peut y avoir de ces sortes de syllogismes (link)
Il n'ya que les farouches et les hautains pour avoir de ces tendresses (link)
Parmi les cultivateurs ameutés, il devait y avoir de ces vassaux qui ... (link)
Il lui faudra continuellement offrir le vin d'honneur, avoir de ces Messieurs des cantons à sa table (link)
Il est souvent donné aux natures supérieures d'avoir de ces intuitions sûres qu'on ne saurait nier (link)
On ne parvient point à ne pas avoir de ces malheurs en province (link)
L'homme politique ne doit pas avoir de ces ressentiments, de ces haines, de ces souvenirs du passé (link)
Il nous faudroit donc avoir de ces citoyens qui ressembliont aux chiens (link)
Si vous pouvez avoir de ces arbres décrits ci-dessus ... il seront tout couverts de boutons à fruit (link)
There are many more: 1, 2, 3, 4, 5, 6, etc.
There are also the literary examples that jlliagre cited in Voltaire and Dumas with the construction être de ces, which is comparable even if it has a slightly different ring (there we could avoid "some" by rendering it "to be among those").
Others feel strongly that that the construction is colloquial. It seems to add a degree of emphasis. They would translate the Astérix quote along these lines:
They've sure got weapons, those Gauls! Those should have been banned by the International Commission of the Helvetii!
I suspect that there are different registers for an essentially versatile (perhaps neutral) construction.
My best interpretation for this is that the colloquial effect results from an uncontextual usage. That is, despite intuition, an author can use a demonstrative without wanting to refer to a specific instance, set, or kind of the thing in question.
This may not be that surprising. It would be a similar phenomenon to that described in the second half of this answer: for discursive reasons, the mere fact of drawing attention to some degree or some kind suggests an extreme degree or extreme kind without further precision required.
Jamais je n'ai vu une telle arme ! I've never seen such a weapon!
(Telle que ... ? Pas d'importance.)
Il fait si froid dehors ! It's so cold outside!
(Si froid que ... ? Pas d'importance.)
Cette arme est tellement dangereuse ! It's so dangerous!
(Telle que ... ? Pas d'importance.)
Ils ont de ces armes ! Look at those weapons they have!
(Quelles armes ... ? Pas d'importance.)
In such a reading, the "...que devraient" addition would not be a restrictive modifier to indicate exactly which weapons, but a non-restrictive modifier expressing a further opinion on just how extraordinary those weapons are.
Other demonstratives without context
To me, the phrase also smacks of the similar (and informal) technique in English of using the demonstrative even when the object has not yet been mentioned. This creates a particular effect:
And then it bit you? It must be one of those half-feral farm dogs.
[apologies to farm dogs]
Such a type of dog doesn't need to have been mentioned before for this to work.
This is also a favourite technique of low-quality magazine writing:
You're all ready for your date tonight. You've painstakingly done your makeup. You've put on that black dress you save for just such occasions. The one with the lace hem.
When Laure says that the nuance of this technique is the same in French, I think that this is what she's referring to. The effect results because the author presupposes that you know what they're talking about. Sometimes this is used to create a false familiarity, as in the magazine copy; sometimes it's just for humorous effect (because it implies that you ought to recognize some category that's actually unlikely); and sometimes it heightens a derogatory effect:
Why, you're one of those cowards, aren't you!
In the Astérix example, the sentence goes on to qualify the secret weapons, which adds context and diminishes the effect of that presupposition — so it could have a literal reading. But it is syntactically possible to read the clarifications after les gaulois as an unrestrictive modifier of ces armes, which would make ces armes one of these contextually underspecified demonstratives and result in the colloquial "emphatic" reading.
P.S. As you've no doubt reflected, the same construction in your other question makes it clear that the colloquial, "uncontextual" usage is found in Astérix. Read that fact into this case however you like!