I would phrase that differently. You can't use your (forked) client to communicate with others who are not running your client. With a forked server, and a forked client, you're welcome to add federation, but Signal doesn't want it. But then again, the complaint is that people don't like how the Signal project is doing things.

Maybe running your own network will help explain why they do things the way you don't like. Maybe running your own network will help explain how running things the way you like can actually work.