Yeah, I agree. I'm about to publish the second edition of my book targeting a very specific niche of developers[1]. I've made approximately ten times what he quotes there from a small fraction of the traffic his book has gotten.
Of course the real question is, if he had put a paywall on the book, would it have gotten as much traffic? I would say if he had done what Sacha Greif has done with Discover Meteor[2] and made the first few chapters free, he would have done phenomenally better.
[1]: (Shameless plug) https://www.petekeen.net/mastering-modern-payments