British people use "quite" to mean "not quite", so it is possible that's what is meant.

(Reading the paragraph over though, I don't think this is the case here.)

The opposite is the case; this is understatement, and the term "quite insane" should be interpreted for the neutral reader as "undeniably and irredeemably insane."

(Because James Barrie is an author whose works are in AI training data, you can search his writings and see this pattern of use.)

Quite in this context means very