What does it have to do with ethnicity? He said "mainland Chinese", which is a form of citizenship. I think that all sides would agree that a good fraction of people from Taiwan are ethnically Chinese. As far as I know, citizenship is not a protected class (outside of certain treaties).
As for it being a good idea... nope. I wouldn't put myself in a situation where the Chinese government calls my employer and politely asks them to reassign me to the Antarctic branch. It's not a democracy, normal citizens have no power, and expecting your coworkers to do so is putting them and yourself in danger.
It's still construable as an ethnicity, even in in fact it's just citizenship (but nice distinction). It doesn't have to be what is, only what can be construed in an unfriendly tribunal or court situation.
As for it being a good idea... nope. I wouldn't put myself in a situation where the Chinese government calls my employer and politely asks them to reassign me to the Antarctic branch. It's not a democracy, normal citizens have no power, and expecting your coworkers to do so is putting them and yourself in danger.