I don't understand why this follows. The standard argument I hear for why women are under-represented (relative to the null hypothesis) is the "pipeline," that at various stages in the process women drop out and it is hard to retain them. In particular, there's the argument that women put their career on hold for their family, which implies that there's a significant number of under-employed senior women.
I'd be very interested to see the number of years of experience vs. level assigned by Facebook, grouped by gender, which would answer whether Facebook's gender balance is really the result of a bunch of 0-years-experience hires. It seems very possible that causality here is the other way around: because women get their code rejected more often, therefore they do not get promoted as much.
As long as Facebook continues to increase the percentage of women it hires, it will have a disproportionately high number of junior female engineers. Intentionally failing to control for that fact is great if you want to profit from the problem of under-representation of women in engineering roles, not so much if you actually want to solve the problem.