I think that Taiwan has a right to self-determination at this point but calling them the “legitimate China” is a bit of a stretch IMO. The only reason the KMT was able to flee and take Taiwan was due to having lots of cash from the US which they basically embezzled.

My great grandfather was in the OSS (CIA precursor) and spent much of the 30s in China. According to him we gave bags of money to the KMT to fight communists which they most absconded with, and bags of cash to the communists to fight the Japanese. So while we may have wanted the KMT to win we also didn’t want Japan to encroach and the communist army was better at fighting them. So what I am saying is the US govt was playing both sides (surprise surprise) and look what happened.