Earlier quoted context omitted.
Non profit but still against federation or anyone running an alternative client. Why?
The good answer: They can't verify the integrity of alternative clients and that they don't leak info The other answer: They've got somewhat of a "we know best" vibe going for them which also comes in play when you see their response to feature requests - e.g. for usernames instead of phone numbers or for "edit message" functionality like Telegram has.
The good answer isn't even a good argument.
The client is open source. People have forked it and used the Signal network. Signal asked them to stop and they did, but there is nothing stopping people from ignoring Signal's request in the future.
This has nothing to do with federation. Signal could federate their network and still request everybody use the official client.