Well, they do want Taiwan back.
Back? They currently own it. No country in the world recognizes them as their own country. They are not in the United Nations. They have no alliances.
If Taiwan declares independence and is recognized... and China invades to stop this. Who can even stop China if they decide to take them? Nobody is coming to their aid. This is a super silly thing to say.
Anyway, you are more wrong that I am.
And in event of PRC attempting to reunify TW, formalism will categorize that as a civil war, not an international one where sovereignty is violated like current RU/UKR conflict. As far as I know, PRC doesn't need UNSC approval to finish an civil war, it's already dejure a legal ongoing war. Plus there's all sorts ambiguity on 3rd parties aiding UKR with lethal aid right now, but aiding TW will be declaration of war with PRC. These are the things that really matter politically when bombs start exploding.
I suppose it makes more sense to say PRC owns "China" including TW in the same way it makes sense to say TW owns "China" including mainland.