But really, I don't think this is that likely, is it?

I think that particular reversal is probably unlikely in general, but I can think of several plausible cases when it would exist.

Suppose that IQ positively impacts both education and income. But education has a negative effect on income, because the more educated someone is, the more they will choose to work on abstract tasks which don't pay as highly. (A salesman earns more than mathematician, say, and the primary function of education is to convince some people that mathematicians are higher status than salesmen.) It looks like the impact of education on income is positive, because of the effect of IQ. (This is basically the same as the reversal effect we discussed, except swapping out sex for IQ.)

Suppose among workers in general, qualification has a positive impact on earnings. For one particular sex at one particular firm, the selection process might be such that qualification has a negative impact on earnings. For small firms in particular, this situation might be likely to arise by chance.