He said "duh, you aren't supposed to finish it - you should only get to the parable about book length, a couple chapters in!"

The parable explores the idea that a reader always has a piece of information you don't want them to have: the amount of remaining pages! If the bad guy dies and you still have 3/4 of the book in your right hand you can be sure it's not over... So as an author what do you do? Add a bunch of Lorem Ipsum? The reader will flip through and figure out where that starts. Splice in an unrelated story? The reader can still find the boundary. So you have to continue the story, but have nothing important happen (e.g., 'the next day Harry Potter ate some toast and watched Friends'). Chris was arguing that GEB talked about doing this, and then actually did it.

The author was clear that he makes his point early on and then just explains it many different ways, so maybe I believe Chris... Or maybe Chris and I are just happy to rationalize our laziness however we can :P

When writing is good, succinct, and to the point, people will finish reading if they're interested in the material. If it's too long, fluffy, repetitive, annoying... people won't. I don't think it's a huge surprise.

More seriously, I agree that writing short books or articles is an important skill. Writing long is easy; conveying the same amount of information concisely is much harder. It is also a sign of respect for busy readers.

I constantly find myself asking LLMs to be shorter, more direct, and more to the point, without fluff. They seem to have a tendency to generate endless streams of words.

My favorite in this regard is Minsky's "Society of Mind" where essentially every chapter/mini-essay is one page, a form which he argued is a good fit for our mental processes.