Those are glorified browsers wrapping the web based clients in dedicated "tabs". AFAIK it looks like Beeper hosts a matrix<->service bridge between those platforms, and their client actually unifies messages in a single inbox. Seems to be different.
Interesting! It would be great though if you could sign up without a Google account. One of the reasons I want something like this is the data collection of all these separate apps. I'm trying to get away from Google :)