The assumption is that these women are just as capable as Y Combinator applicants, but that the program is biased against them. Even if you believe this, any time you restrict the applicant pool arbitrarily (by gender/etc), you will lose some of the best applicants. For example, you will likely at least double the great applicants by allowing 40+ y/o males to enter as well. Hence, it just doesn't make sense as an investor to participate in "XX Combinator."
I still don't get that this means 'lower the barrier' though. Doesn't it mean raise one impenetrable barrier, i.e. XY? If the same number of XXs exist then the number will still be the same but fewer people will apply. If more women apply as a result of good PR about it being woman-friendly then they still may not get accepted on the same grounds as any other.