I confess I'm one of those and must accept your equanimity, for I assume you have actually read the book!
But as to the lack of expert qualifications of Guzey, I feel his analysis is a forensic not a counterfactual one. If you can't trust Walker to present other people's findings accurately, why should you trust his conclusions?
And it does go beyond gotcha !
Guzey accuses Walker of hurting patients and cites sleep specialist's testimony that in obsessing over not meeting Walker's criteria of health inducing sleep, their patients blame themselves, not the theory. More consequential (and new to me) is the harm Guzey claims is incurred by misleading depressed patients who might otherwise respond to deliberate sleep deprivation. Like, who knew?
Throughout medicine, and biology in general, the best study of man is not man himself, but man in disease .. both natural and deliberately induced. These are the exceptions that prove the rule.