Well, if Biden should be characterized on how he views Princeton today versus decades ago, perhaps he ought not characterize Alito on an organization he was a member of decades ago.
I may have been a Girl Scout decades ago, but that doesn't mean I would want to be a part of them now. They have changed so much. Not always for the better.
But you have it reversed. Biden thinks Princeton (the Girl Scouts in your analogy) is okay today, but not in the 70s and earlier. Biden wants to judge Alito based not on how the Girl Scouts are today, but on how they were when he was a member (so to speak).
Biden has made two sets of comments, each very distinct-- the nature of Princeton decades ago and the nature of Princeton more recently. Those statements can't be in conflict since they aren't about the same thing at all. He's not judging Alito on comments, either, but a membership.