Taiwan is opposed to mainland China at least as much as the US. Sure they are closer, but mainland China is also threatening to annex Taiwan.
I think it's little bit more complicated for people with Chinese ethnicity (no matter whether from China or Taiwan) than you paint it, seems he had zero problems to have affair with mainland Chinese woman rather than Taiwanese (and he certainly knew she is not Singaporean unless he is very stupid), world ain't black and white as some US politicians trying to make it.
It's very complicated, I just tried my best in a few words.