Well, as it seems, they've existed as minimum in both in Theravada and Dharmaguptaka. That means they are at least ~2300 years old ,)
And no one can ever prove that Buddha didn't establish them. It seems likely that they were established by him, because they are very old.
What is more - the story about Ananda asking Buddha about women ordination is accepted not only by Theravada school. That means that this is true, that Buddha didn't want to establish bhikkhuni sangha. And from Vinaya we know that it was that very story when he established garudhammas.