It would be plausible, but if you examine the historical enrollment numbers it's clear that CMU expanded their CS program around the exact time that their female representation went to 50%. In essence they created more spots reserved for women, but still admitted the same number of men each year.

You'd also have to ignore that similar caliber schools have the same 20-30% ratio so CMU would have to be doing something special that MIT, Stanford et. al aren't in its applicant recruiting/marketing - that's a tough one to believe.