My guess is that the OP was a bit imprecise, and that he kept some assignments the same as previous years. If 3 assignments are same-same-different, and a student gets grades A-A-F, that could be what they call a 'tell' :)
On second reading, the OP was being sarcastic, and derisively noting that if he changed his assignments, he never would have caught the cheaters in the first place. So he rejects one form of making cheating useless (making traditional assignments, but changing them up each year) and embraces another (choosing different types of assignments that are structurally not cheatable).
I believe the intended reading is that he does "change" them, by adjusting the font, etc, as mentioned just earlier. His colleague was suggesting that he write entirely new assignments each year.
The author misunderstood what his colleague meant (Or at least, only does a half-measure), which is why he says that "He is" changing them.
No the suggestion was not to create entirely new assignments every year. This is problematic, not because it takes time, but because is it hard to "debug" the assignment, and make it to be not too hard, not too easy, and not ambiguous. You cannot know this before actually giving the assignment out to students.
The senior professor was just suggesting to change the numbers, or small elements of the assignment. A thing that I was doing already.
