Maybe, I guess it all depends on whether they agree on how transferable said license is, and whether courts or even just DMCA enforcers at a host or registar will agree with that (DMCA safe-harbor provisions encourage providers to act preemptively rather than duke it out in court else they risk losing protection against liability).
Obviously one's own server's TOS guarantees they have the license to host their users' content, but it might not be as clear for other federated servers, since they have their own TOS that protects them in that event, but if the user didn't sign up there, they aren't agreeing to them, and they could even reject it and still have their content federated there whether they like it or not. Seems like it could potentially be a dicey grey area if they chose to pursue federation as copyright infringement. Especially if they copyrighted their comments and posts officially.
ml might be a hard one to sell for people, if servers would defederate from ml that would be good but I think many servers right now are afraid of it backfiring like how Beehaw defederated lemmy.world and sh.itjust.works and killed 90% of their interaction and 100% of their network effect overnight.
Of course I do think things are different now and if big servers defederated ML, ML would be the one to lose out, not the bigger servers. Only reason people still use the Bigger ML communities is because they continue to federate, if they weren't people would use the other ones on places like lemmy.world, sh.itjust.works, lemm.ee, and programming.dev.