Some of the group’s members decided to form an offshoot group in which students could share obscene, “R-rated” memes, a student told the Crimson. The founders of the messaging group demanded that students post provocative memes in the main group chat to gain admittance to the smaller group.
But from wording later, the examples given seem to be from the private group? I'm also unclear if the private group some some offshoot of the main group within Facebook (that is, whether it's some Facebook sub-group feature, obviously they are members of the main group).
In any case, Harvard says they reserve the right to reject admission based on "honesty, maturity, or moral character" so if this information is brought to their attention, then it falls under what they consider valid acceptance criteria. It's no different than if they Google searched the applicant initially and found verifiable information that put the student in the same light.
Except that the premise of Cards Against Humanity is that you are all opting in to the situation where you expect that, and usually an a somewhat private setting.
The equivalent to the CAH suggestion would be where some private group of people indicated that to become members, you had to go to a public cafe and play CAH loudly in public. Were that Cafe to get mad at you and ban you from their premises, getting upset at that outcome is rather silly. You either knew that was a possibly outcome, or you should have known. In any case, that Cafe is well within their rights to decide they want nothing to do with you anymore.