The usual pattern is like that Alex Norris comic:
"you do not fit in here"
"okay we will make our own place"
"why are you excluding us"
"oh no"
Except here they can stay connected with the broader Atmosphere but engage on their own terms.
I'm not as familiar with ATProto as ActivityPub, but following someone from another Mastodon instance, for example, is seamless as long as I'm logged in to the account I have on my home instance.
I can't comment, follow, like, or anything like that without getting the "Sign in or create your account to join the cookout!" popup. I wasn't trying to cause problems or get downvoted, this is just the the first non-BlueSky PDS I've ever come across and was curious to see the federation work.
FWIW— I have also not heard anything even remotely close to this at all from anyone using either service.