The problem is that Claudine Gay was not sacked, she was allowed to resign as president and is still, at this moment, a professor at Harvard. Here is her faculty web page:

https://aaas.fas.harvard.edu/people/claudine-gay

So Harvard employs, as a full professor, someone whose Ph.D. thesis contained loads of plagiarism (I’ve seen the evidence, it’s not contestable). A similar offense on the part of the students who sit in her classroom, according to Harvard’s own rules, could lead to expulsion.

EDIT: Also, as pointed out in a comment below, Prof. Gay’s Ph.D. is from Harvard. It was not revoked.

> Recently a Harvard president, Claudine Gay was sacked.

But Gay's PhD was not revoked.