I think this author is talking about a subset of non-fiction books, and we may as well call them "non-fiction idea" books, where the writer has a single idea they want to convince you of. On Tyranny does that. And sure, maybe books like Checklist Manifesto and The Goal could have been reduced to a hundred tight pages.

The hundred page idea definitely doesn't apply to non-fiction in general though. I can't imagine a history book or a biography covering what they need in a hundred pages. I don't think Chip War could be reduced.