[corvec]

It's more like:

- 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old versions of Signal are OSS, but frequently updates are only shared after a long delay - in some cases over a year out of date, if my memory serves me.