Some caveats to think about for that particular book in this blog post: https://guzey.com/books/why-we-sleep/ (I'm not a researcher in the area, so not sure how valid either side of this is!)
Point number one of his list of 'egregious' errors is the idea that shorter sleep does not imply shorter life span, but all of the recently published meta-analyses make that connection convincingly (for the population sleeping <6 hrs). Anyone can look up these studies. And Walker does not advise people to sleep 9+ hrs (which is also thought to be a risk group on a population level). The author of the piece from the outset is quite evidently arguing in bad faith.
There is criticism to be had on Walker, he overstimulates his readers with fear-inducing statements which can definitely backfire.