Discrimination on the part of gatekeepers isn't the only thing that can keep minorities out. A culture that tells them not to try doing it because it isn't their assigned role, a fear of being the only one of their group among a majority of another group, and earlier discrimination that prevents them from getting qualifications but isn't the fault of the gatekeeper could all be responsible. Given the information I know and what has been brought up in this discussion, University mathematicians could have been perfectly willing to hire women.

This is just the good old pipeline fallacy all over again.

The fact you like to call it fallacy doesn't make it one.