This is true, however he is trying to explain the idea to multiple people, so he's explaining his ideas in multiple ways.

So no, it could not be a long form blog post.

Is the idea that complicated? Can it not clearly and concisely be explained in a way that makes sense to a large audience? We see more complex ideas explained more concisely all the time.

It's not just Cal Newport, it's self-help and productivity authors in general. They have an idea, and then they repeat it over and over again until they have a book.

Maybe the repetition benefits the reader. Maybe it's easier to internalize an idea when it is hammered into your skull. Or maybe books are just more profitable than blog posts.

I find this even in well-respected and oft-recommended but relatively light non-fic outside the self help genre. Sagan's A Demon Haunted World and Cialdini's Influence both struck me as being twice as long as they had any reason to be, but they're nigh-classics in certain circles. It makes me reluctant to read non-fic, which tends either to be very specialized and ultra-dense (i.e. beyond my reach with a reasonable time investment) or fluffy trash.

However, better small-scale (paragraph to paragraph) structure of the text and somewhat higher info-density, or simply being more entertainingly-written, can go a long way. See Graeber's Debt for the former and Fussell's Class for the latter.

It's not just exposition, it's rhetoric. Self-help books take a lot of words because they're exhorting you to an ideology, not just explaining things.