- If you use our client you can use our servers - If you don't use our client, you can't use our servers, but you can use any other server
It's like, technically it's sometimes[1] OSS, but they don't care about actually being FOSS in practice. If I can't fork the software, add or remove a feature and keep using the software's other features, it hasn't hit the bare minimum to be called FOSS, IMO.
1 - Most old versions of Signal are OSS, but frequently updates are only shared after a long delay - in some cases over a year out of date, if my memory serves me.
(I like Matrix and will always sound like I'm dunking on them because of the implications of Nebuchadnezzar, and, before that, of opt-in E2EE; they're doing mostly the best they can with a tough hand to play.)