He was* harebrained (unwise), [a fact] extremely obvious by what he said (by his words), despite his education. This stupidity (denseness) of his instantly became apparent to all (was noted by all).
* The tense used here is one for past but, in this context, unless a mistake, it can either mean that a) that person is no more or b) that it wasn't the "norm", it was a one time thing. So, either "he behaved" that way or "he was" that way but he is dead now. Or, as I said, it's a mistake.