I suppose it's one of those statements that says a good deal in context and rather less outside it. 'Scientism' usually refers to a belief in the universal applicability of the tools of science in understanding the world. It is so understood by two camps, one who views it as an intellectual failing, the other a virtue. Wilson's point is that the latter camp should not cede any ground to the former -- not even terminological ground.

Edit: by context here I don't mean the book in particular. More like, reading too much contemporary philosophy.