I've been actually wondering the same thing. I recently read a whole book on language learning (How Languages are Learned). It seems like from the current knowledge the field has Pablo isn't right that you need 100% comprehensible input. It might work for some people, but for the majority its better and faster to learn grammar well.
That's not to say we need to go back to the "old school" methods, but it seems like learning through context (which CI definitely helps with) along with grammar and some memorization yield the best results. Pablo is mostly drawing off the theories of a few researchers with not too much tangible data to back up the theories.
Also like to reiterate I'm not means an expert though and love what Pablo is doing! Like others have said, I think mixing and matching is the best.
Yes, the ebook is for sale. (A free download would be pirated.)
https://play.google.com/store/books/details?id=5PadBgAAQBAJ
https://www.amazon.com/How-Languages-are-Learned-4th-ebook/dp/B00BHHIYBC