(L'Étranger de Camus) "tant" in "Ils ne s’en apercevaient pas tant ils étaient absorbés dans leurs pensées."

Question

The question is on tant in this quote from L'Étranger by Camus.

De temps en temps seulement, j’entendais un bruit singulier et je ne pouvais comprendre ce qu’il était. À la longue, j’ai fini par deviner que quelques-uns d’entre les vieillards suçaient l’intérieur de leurs joues et laissaient échapper ces clappements bizarres. Ils ne s’en apercevaient pas tant ils étaient absorbés dans leurs pensées.

Questions

1. Is tant an adverb modifying absorbés?

2. If yes to (1), then is it common in French to run two main clauses together without any conjunction (or even so much as a comma) between them? Or is that something we see only with tant separating them as in the quoted sentence?

3. Or am I wrong to see two main clauses in the sentence? Perhaps is there an implicit que (as below) so that the first clause is actually subordinate?

[Qu’]ils ne s’en apercevaient pas tant ils étaient absorbés dans leurs pensées.

Background

You don't need to read this background to answer the question.

When I first saw the French sentence I thought it meant:

They didn't notice it so much, they were absorbed in their thoughts.

But four translators (as below) have read it per (1).

They were so lost in their thoughts that they weren't even aware of it.

They were so much absorbed in their thoughts that they didn’t know what they were up to.

Sie merkten das gar nicht, so sehr waren sie mit ihren Gedanken beschäftigt.

Sie waren so sehr in Gedanken versunken, daß sie es nicht merkten.

Answer 1

• Both translations: They were so lost in their thoughts that they weren't even aware of it. and They were so much absorbed in their thoughts that they didn’t know what they were up to. are very correct.

• No, we can not put a comma , before or after tant.

• No, as the phrase is, we can not add que to tant here. The original phrase can be reworded as follows:

  Ils ne s’en apercevaient pas tellement ils étaient absorbés dans leurs pensées.

• The implicit que you are talking about can be used under the condition you change the position of MC and SC as follows (MC + tant ...que + SC):

  Ils étaient tant absorbés dans leurs pensées qu'ils ne s’en apercevaient pas.

  Equivalent to:

  Ils étaient tellement absorbés dans leurs pensées qu'ils ne s’en apercevaient pas.

• Additional information: tant is called adverbe corrélatif and it expresses the intensity (as in your example) or quantity (in some cases)