You are right. That said, even if the booster is supposed to stop Delta, Lambda is already here in a small way.
That will be the next shot after the next booster.
Considering any new mutation needs to be identified, a reformulated vax created, tested, approved, mass produced and then given out to hundreds of millions of people - a process that could take 6-9 months+ more than enough time for a new mutation to form it’s become a pointless exercise.