the connecting with a majority of people using the same closed source platform
The platform is open, including the part that connects to other closed source platforms. It's just Matrix and open source bridges after all. And making the client app closed souce doesn't help with any of that.
I'm sorry if I'm a bit pedantic about this, but it seems like you're describing an upside to closed source software that's just not there.
Nothing about what you just wrote has anything to do with closed source software though. You could just as well say that closed source helps them predict the future or draw shinier unicorns. It doesn't!
Maybe you mean tightly coupled, stripped-down, preconfigured or vertically integrated, but you can do that just as well with open source software. No one is forcing them to make a general purpose chat app or offer the ability to choose a different server. It's just a matter of being able to see, verify and modify the code.
This is the only thing that comes close imo. But they stated specifically that they don't want to make money with the chat app itself, so it doesn't really work as a justification. They could easily offer server-side premium features or create a closed source premium-only version or extension, it's no reason to make the base app closed source.
They don't have to do that, and they don't afaik. Matrix itself can do proper e2ee just fine, and Beeper is pretty open about the fact that bridges hosted by them have to break e2ee to translate between platforms. They'd only need theater if their closed source app actually has some bad code in it, which is kind of my point.
Again: Their Matrix server and bridges are open source right now, and it wouldn't stop them from doing what you're describing.
I just can't help it. π