Hi, I'm the author of the piece. I cover this objection specifically in Section 14 of the essay.

tldr: Walker claims his book is scientifically accurate (I quote him in the introduction), he cites it in his academic papers (see section 14) and therefore he seems to have an obligation to be precise in his language -- some of the book's misinformation has already made its way into the scientific literature. As I note in the introduction, this depression thing is not a single incident (of course there will be slips in a >300 page book), but instead a consistent pattern of exaggeration and scaremongering.