Hmm… 
In the early times, there were many highly accomplished bhikkhunis too. Plenty of them were stream-enterers even before they ordained. So getting teachings wouldn’t really have been a problem.
Don’t forget that this rule has to be followed even by anagamis and arahants. Or very accomplished practitioners well on the way to high attainments.
But in any case, bhikkhunis have to attend the forthnightly uposatha, so there are several rules prohibiting long seclusions. And there is the rule about not spending the night alone, not wandering between villages alone, etc. Seclusion is difficult for bhikkhunis…