The Gemoro says (Nedorim 83a, 6 lines up) regarding an annulled nezirus that Chatas Ho'of can be brought, because it is brought for a sofek.
Why can Olas Ho'of not be brought - it can be brought as a nedovo!
Meir Eliezer Bergman
I actually had the same Kashya. On second thoughts however, the answer is simple. What the Gemara means is (not that the woman is forbidden to bring an Olah - the Gemara is not speaking in a case where the woman has already designated the bird - but) that she is not obligated to bring one as 'Rashi' specifically states.
Be'virchas kol tuv.