That’s enlightening. It links to an article about self hosting a relay, which explains that, as I suspected, a relay does not have to mirror the entire network. It also seems that using a relay at all is an optional optimization.
It looks like the BlueSky AppView is not (yet?) open source. I wonder why nobody has built an alternative yet.
This looks like it’s conflating service providers and clients. Thunderbird doesn’t provide email accounts to the public as far as I know.