Do those other options actually exist within China? I thought they were blocked and that's why WeChat is dominate.
I think you're talking about domain fronting (https://en.wikipedia.org/wiki/Domain_fronting). IIRC, Google, Amazon, and MS all blocked it because they were afraid of getting blocked. Though I also vaguely recall that Russia was blocking big blocks of AWS IPs for a time to try to stamp out Telegram.
I was in China recently, and Signal actually seemed to work on a local wifi network (with cellular data disabled), so maybe they've got some new circumvention technology in place. I didn't thoroughly try to test it since 99% of the time I just used data roaming on a foreign carrier to avoid the firewall.