"many of them use Signal" - where is this conclusion come from? They have WeChat. I've never seen any of my Chinese friends using Signal.
They probably tried to say that most of signal users use third party keyboards.
https://www.nbcnews.com/tech/security/trump-bans-wechat-some...
Note that even for Chinese people in the US (myself included), Signal was in my experience a pretty minor choice when Trump was trying to ban WeChat. Whatsapp, Line, etc were among the favorite alternatives.