Technology

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.

differentiate above the competition [...] charging for it

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.

security theatre

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.

Expanding to selling some user metadata, or sniffing the bridges, would be an extra

Again: Their Matrix server and bridges are open source right now, and it wouldn't stop them from doing what you're describing.

Too pedantic 😉

I just can't help it. 😜