That's a trademark (or perhaps trade dress) issue because it had "Signal" in the name.
They (the forkers) were stupid to induce brand confusion like that.
Signal is GPL free software; you can fork it and release it with any API URLs you like in the source or binary. It is perfectly legal to fork Signal and remove the user-hostile expiration timer, the image scaling enshittifier that happens on upload, and leave the signal.org API URLs completely unchanged. You just can't call it Signal (or LibreSignal, or BetterSignal, or ExtraSignal, or whatever) when distributing it.
The API ToS is what applies to end users who connect to the API. It says nothing about what software you are allowed to use to do so.
Signal would prefer to pretend that they get to choose what tools their users get to use to consume their service. Unfortunately for them (but good for us), they don't. It's the web and such a restriction would be insane.
Don't confuse software and services. They aren't the same.