In this experiment, the employers picked between two candidates, one male and one female. If you pick the male on the basis that men tend to have a higher variance of performance, you will end up with a significant underperformance as much as you end up with a significant overperformance. In any case, the task was not to achieve the highest absolute performance - it was to choose the better of two candidates.

In other words, although the variance explanation might give a rational reason for hiring men over women in the real world (although I am very skeptical) it certainly doesn't give a rational reason for hiring men over women in this experiment. Your criticism does not apply.