Umm… I was not so very clear perhaps. The idea would still be that user accounts as well as forums all contain their domain name, as their site of origin rather than a location identifier. Just that the host could change to any other domain (after negociation with the new host, that is). So it’s not about domains being tied to specific hosts/IPs but entities being tied to domains. It would be up for design discussion if that identifier should change or not, iin the case of a migration. The idea would be to give entities the ability to roam or be resurrected from any federated copy in case they are dissatisfied with the policies of their hosts, or in the event a domain gets taken down by authoritrian actors. (That’s why this actually is off-topic here)
From my glance into the ActivityPub doc, I concluded that it’s really only about the data exchange protocol, yet I might have overlooked something as I never had an in-depth talk with people who implement the thing. Yet, just because many do it in a certain way does not mean to me that this is written in stone somewhere. :-)