> I haven't heard a single reason for holding off federation from Moxie (and the "best" reason I currently can come up with is that he has issues letting go of "his baby", other reasons being more nefarious).

I mean, there's no need to speculate here. There's an entire blog post on signal written about why they chose not to federate just a quick google search for 'signal federation' away, and the reasons, whether you agree with them or not, are pretty solid and sound.

https://signal.org/blog/the-ecosystem-is-moving/