This misses the point. The parent isn’t talking about just building a few chat apps. They are talking about building a chat app which can have basically any ms office thing embedded inside it.
Office is going the other direction and moving more of the "full-featured" Office components to React Native and sharing them with the web. Teams was built as a quick spike by the SharePoint team to prove they could slackingly build a chat client. It might not be a surprise if it moves to React Native in the future.