Even if you don't find the particular word in a dictionary, a "NonXXXX" is invariably something which superficially looks like (often, was supposed to be) an XXXX, but missing with some (usually, essential) characteristic. I would say it's a "productive" way of generating new words.
Probably the primary characteristic of lessons is that they are learned. So in the absence of any further context, I'd interpret a nonlesson as something "intended" to impart knowledge, but which fails to do so for some reason.
I rule out the possibility that the lesson wasn't learned because the student failed to pay attention, because that's not necessarily a fault of the [non]lesson itself. But I can't really speculate on why OP's particular nonlesson is thus labelled.