There is essential information about this pronoun on this site.
In this context "que" has the function of "attribut du sujet"; "que" stands for "pays touristiques incontournables"; in a clause where no replacement by a pronoun is used this is the function of "pays touristiques incontournable";
- L'Autriche et l'Italie sont des pays touristiques incontournables.
So, when "que" is put in the place of that (pays touristiques incontournables) it's got the same function.
Why has been "que" designated for this function seems to be that if "qui" had been chosen, this pronoun having also the function of a subject there wouldn't have been a way to tell sometimes whether who or what represented "qui" was attributed something or doing the action. As "que" can only be COD and that "être" has no COD, there is no risk of confusion.
It might be useful to consider that in English there is no difference;
- The opposite, what these assertions are, he refutes.
(However, the best translations into English do not necessarily have to be by means of "what"; this pronoun can sometimes be done away with for the better of the translation.)