You are absolutely correct - WeChat is what it is because of the great firewall. It's not that China blocked other messaging apps (though they do now), it's that China crippled the web and as a result users have been driven into these apps.
You can see it in other ways as well - In the US people generally shop on the web, but in China people shop in an app and the web version is not as good.
I guess some might call WeChat payments and mini programs innovative, but they're not very impressive technically. Take mini programs, they are basically a web app hosted within WeChat. It's kinda cool, like Facebook apps were kinda cool, but neither holds a candle to the power of a real browser. So Facebook apps were kind of a fad and then we got bored and went back to the open web. But Chinese people can't do that. It's not that they can't have web browsers, it's that they don't have any web innovation. It's not surprising, the web is so locked down with license requirements to even put up a simple web site. And so there's no good hosting options, which in turn makes it more productive just to stay on WeChat and sell your wares though Alibaba.
It's all in the one app because there is the state in the background intervening to make it so. But dictatorships are not market validation.