I think it's clarified if you think in terms of sets.

If you were to set 21 or 21+ as a superset (the necessary condition) and legal as the subset (sufficient condition). Then is means that there may be a case where someone is not in the "legal to buy" set, but still within the 21 or older set.

But in strict context of the passage, that is not what the logic is saying. In the sentence it says that ANYONE (so all) 21 or 21+ can buy. So 21 and 21+ has to be the sufficient condition.