Via Tim McDaniel, this Saturday Morning Breakfast Cereal:
(Maybe I should create a new postings category Stereotypes of Linguists.)
The professor believes that all pedants are condescending, so that condescending pedant is a redundant, pleonastic, phrase. But quite likely the student intends condescending to be a appositive rather than intersective modifier (see the pilotless drone discussion here) — reinforcing the component of condescension in pedant, rather than narrowing the reference of the noun. From the pilotless drone posting:
You might think that even the appositive reading of “pilotless drones” would be stupid, since drones are all pilotless. But look at the explicitly appositive version: “drones, which are pilotless”. This isn’t stupid at all; it REMINDS us, in a helpful way, that drones are pilotless. In general, even when the denotation of Adj is included within the denotation of N, appositive Adj N can do useful discourse work. As a bonus, since intersective Adj N is stupid in this situation, the potential ambiguity is eliminated in practice, in favor of the appositive reading.
(Of course, labeling condescending pedant as a redundancy is itself condescending pedantry, so the professor’s last sentence has the flavor, if not the actual form, of self-referentiality.)