Walker has clearly made a huge name for himself and sold a significant number of books by skewing the evidence to make his thesis sound more substantial than it is
It's less about the specific content, and more about continuing to push something without good science behind it.
For example: I think X is true, I cherry pick data to back up X and publish a book on it. This book gets a lot of attention and is good for my career, but it's based on bad science and misleads people. This is a theory I identify myself with along with my success and my own career, if its ideas are invalidated that reflects negatively on me and my abilities (and potentially the ability to support myself). There's an incentive to rationalize or continue pushing the bad science which slows down figuring out what's really true. This has happened a ton of times throughout history.
In this specific situation the benign case is people sleep more and it doesn't matter (or it helps), in the pathological case people are anxious about their lack of sleep and it negatively effects their life or they sleep more than they personally need and it turns out over-sleeping is co-morbid with depression and causes other issues.
In the general case popularizing bad science has knock on effects that make it harder for people to get funding to study ideas the contradict the popular, but incorrect zeitgeist of what's commonly thought to be 'true'. It can also lead people down rabbit holes that can take a long time to crawl out of and make it take longer for people to understand what is actually true.
You can't just hand wave this away by saying "I know X is true, therefore it doesn't matter if the data is a little problematic. X is true and it's good for people to know that". A lot of times X turns out not to be true, not to be entirely true, etc.
I don't know if you can call it an agenda but I think there's something like a 'wellness' or moderation bias in these fields. The idea that you need to balance out hard work, slow down, and so on.
For example, as it turns out there is little evidence that stretching actually does anything, yet a lot of experts used to recommend it for decades. Same with nutrition, 'balanced diets', workouts at low heart rates and so on.
What do you mean that stretching doesn't do anything? Any links to such evidence? It's a pretty simple experiment to stretch every day for a month and measure your flexibility gains, so that's quite a bold claim. Do you mean it has no direct impact on your health?
it actually has negative impact on health and injury in people who do static stretches in particular is significantly increased. (as is in people with high levels of flexibility in general)
Nowadays stretches as a warmup are not recommended any more.
