But college is not the be-all and end-all of someone's political evolution. Buying a home and having a family also influences someone's political opinions, broadly in the more conservative direction.
What I think sounds plausible is that the rise of women pursuing post-graduate degrees brings down birth rates. If a woman isn't done with her academic pursuits until she's 30, she'll unquestionably be less likely to have kids as a matter of biology.
Moreover, if it's extremely difficult to buy a house and start a family until you're in your mid-30s, that's going to keep people more liberal for longer.
So I don't think the basic dynamics have changed, but the timelines have. When people have a house and family, they have things to lose and act more self-interested. When they don't, they don't, they're much happier to entertain proposals for vast societal change.