The fact that you never heard of it doesn't mean anything. But you not acknowledging it doesn't surprise me either. It has happened, does happen, and is happening. Several of my friends have received threats like this when they were in college. Of course, the wording is intentionally obtuse, but the point and impact is unmistakable.
I cannot read into this any impugned measure of a person's performance or credibility. I see a comment that refers to a sampling population of one that is statistically insignificant.
I experienced a disproportionate number of bad apples in the Humanities realm of college in the 70's and 80's in both undergrad and graduate work. Multiple choice keeps profs honest but anything other than an absolute structured response population is an opportunity to punish dissent. Then there is the problem that multiple choice testing does not provide a good measure of comprehension...unless they are structured in a manner that goes into both depth and conceptual relationships; they can be very poor indicators of anything but the ability to take notes.
Then there were the football players who rarely passed a test and got C's and all preferential teacher focus in the class time holding the rest of us back....didn't get anywhere with that one either...but I did learn a valuable lesson.
Integrity does not exist in a vacuum, it is earned by testing and is proven to exist.