For all he knows, the students went to a site with study materials to prep for a test. His policy of disallowing students to refer to prior tests was not even explicit in the syllabus, but behind a link. If this policy were clear upfront - absolutely, reference to outside materials is inappropriate and there should be consequences. But I don’t think it’s reasonable to impose these consequences after the fact based on a reference to a reference.

Based on his post, my assessment is that the prof is lazy and an asshole. Maybe there are other relevant factors that would change my analysis, but I have to think this is the version of the story that puts the prof in the most favorable possible light, so….