I think Keen makes two points. One is (as it often is the case in Debunking Economics) that this is "surprising" for students of economics. He cites various textbooks, like Mankiw. If economists know these things, why do they lie to students? It is indoctrination, plain and simple.

The second point, IIRC, is that the Cournot model of perfect competition is mathematically wrong. The assumptions that no firm can influence the price and decreasing demand curve are contradictory, regardless whether or not these provide a good model for the real world.