Like having some portals or borders which lead to the next server

you don't need federation to do that though... the server could Just™ send a packet to the client to redirect it to another server. minecraft for example already has this functionality and nearly all "large" servers make use of it.

honestly all use cases people think of for federated (in the AP way) game servers just feel like an attempt to cram federation as a buzzword anywhere they can think of "because its the next best thing trust me", similar to many other bubbles (NFTs, AI)