just because it isn’t open source

You’re going to want to support projects that share their source with the world. Otherwise, how do you know what you’re actually using? You might want to give a read through discord’s TOS and make sure you’re comfortable with what they’re doing.

technically matrix is mainly made/maintained by one company who made it at the same time, they don't own it and there's a separate entity the matrix foundation which is non profit which owns the actual spec and all that stuff but the majority of stuff done in the spec was by element/new vector

Right. If it’s open source, the public owns it. You own it, as does New Vector and the Matrix foundation. You run the servers, or your friends do. You decide the terms and conditions. You extend the server or configure it in the way you like.

The difference here is that Discord is a black box operated by one company. Matrix is an open box operated by you or people you trust.