Their rules don't state that they need to. Had they chosen to enforce their ethics code internally and take internal action against the students, they'd have be

Their rules don't state that they need to. Had they chosen to enforce their ethics code internally and take internal action against the students, they'd have been within the bounds of that policy. That said, they didn't do that: they used the DMCA. Their use of the DMCA is a strange choice in my opinion, but is likewise valid if the repos contain original work created by the university as part of the problem set.