The ideological makeup of Taiwan has nothing to do with whether or not they are entitled to sovereignty from a diplomatic perspective. International relations isn’t about right and wrong. In fact, the KMT and CPC are in agreement in maintaining the status quo - the KMT and the CPC work together to oppose any attempts at renouncing claims to mainland China by Taiwan and formally becoming the Republic of Taiwan.
Sovereignty as a concept in international relations stems from the Treaty of Westphalia, in that each nation has an absolute say within its own borders and that said borders are inviolable.
Being a sovereign nation is when you don’t have a seat in the UN and most sovereign nations refuse to recognize you as an independent nation.
I really don’t think this is the view people on the left should hold. Someone could say the same thing about many nations or groups that don’t have a seat in the UN and aren’t recognised but are still supported by communists and anarchists.
TBH I don’t think “legitimacy” matters. They function as an independent country. They issue passports, and flights between them and the mainland function as international flights despite both countries making up legal mumbo jumbo that calls it “cross-strait travel”. There are countries with more widespread “legitimate” recognition that are functionally less of a nationstate than Taiwan.