"Who at Google decided to censor American comments on American videos hosted in America by an American platform that is already banned in China?"

Probably no individual. There are enough Chinese pro-nationalists using YouTube to generate noticeable signal if they all, independently based on their political creed or as an organized brigade, decide to start flagging posts. Once the flagging begins, the relative rarity of the characters in question combined against the flagging signal would generate a Bayesian prior that the word in question would tend to get flagged, and would preemptively start killing those comments.

This is one of the ways to train an automatic moderation system that is capable of discovering novel words the community decides are swears, and brigading is a known pathology that those systems are susceptible to.