"Walker’s book has likely wasted thousands of hours of life and worsened the health of people who read it and took its recommendations at face value."
First part is dubious but second part, seriously? How can extra of sleep "worsen" the health of people?
People for whom sleep deprivation therapy would be helpful but who decide to avoid it due to Walker will have their health worsened by the book.
For a bit more speculative answer to this, see Section 14.1: https://guzey.com/books/why-we-sleep/#appendix-common-object...
There are good points in your analysis but shall we discount yours entirely as well for faults such as these?
There are measures to alleviate depression beyond sleep restriction, including therapy, e.g. CBT, shown to be highly effective. A case of one not being prescribed a brand of experimental therapy like sleep restriction is not tantamount to withholding therapy "someone needs".
If that were the case you ought to hold the vast majority of psychiatrists to that standard who opt to prescribe a cocktail of medication, not sleep restriction, which is comparatively uncommon as a first measure.
And most importantly, because in such a scenario that sleep deprivation were to be prescribed, it would not necessarily be eschewed over a fear of the health effects of sleep deprivation in the short-term, a fear strongly presumed to supplant for your point to stand. It's not uncommon for patients to consume medications with negative side-effects, such as those that cause liver, kidney damage.
By the same token, being underslept makes you feel like shit, makes it difficult to focus, increases the risk of injury at work, on the road, etc.