It seems like you need to have been in your current alliance for over one week before the 7-star celebration event started to be eligible for rewards. This was never stated in the livestream, patch notes, or forum posts.
The only response we have so far is from this thread in which Kabam Miike only references exploitations.
https://forums.playcontestofchampions.com/en/discussion/335568/can-kabam-remove-those-timer-for-alliance-event#latest@Kabam Miike @Kabam Jax Will there be some sort of compensation to those players who participated in the event and become eligible during the event, but now can't collect the rewards?