Talking to a Chinese-speaking coworker, my understanding had been that speech to text was very popular in China, especially on phones?

Yes it is.

Tangent: Anthropic's CEO worked on Mandarin speech recognition at Baidu: https://arxiv.org/abs/1512.02595

OK, but it doesn't work in situations where you don't want others to hear what you type. And in places with lots of noise.

In English I use speech to text a lot (including completely substituting it for typing for a year when I had wrist issues). If your microphone is right next to your mouth and you're using good software it works in surprisingly noisy environments: I can talk quietly directly into my mic on the subway without issues. And when I'm at my desk I use a boom mic next to my mouth, with similar benefits.

Quiet environments were, for me, more of an issue: it combines very poorly with open plan offices since you bother the people around you.