I'm pretty sure it's to prevent summoners from hoping alliance to alliance to claim the rewards. If you start a new alliance, it should work since none of the rewards are claimed.
Dont you need to wait like 14 days or something to claim alliance related rewards when hopping alliances?
How about my .I am coming back to the game after sometime & saw this enlistment event.Now i have this query will i get the alliance milestones when i join a alliance & by contributing towards it..Will i get those milestones that are already achieved/ claimed by the alliance before i join them? Or not ?