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' :)
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.