Later he cautions against adding 'very', so he's consistent at least.

what about slightly difficult. It's basically a language denuded of gradation.

I think absolutism on this point is a bad idea. But I do think that many people, including myself, overuse qualifications in contexts where they don't add anything. In the example of a close match, what distinguishes a "very close match" from a "close match"? Better to omit the "very" unless you can quantify it, or otherwise make it clear what it adds.