The gist of what he is saying for people he'd most likely want to reach but can't for the very same reason he's adressing with his lengthy post: When reading a long, carefully arranged cohesive body of (text)work, it does not help to be distracted. With ubiquitous access to seemingly useful resources, it is very easy to give into such and deviate from the intended reading material. Book publishers don't seem to get this when devising their solutions for the declining book readership problem.

Another note by me: Good writing is very similar to good programming/programming language design/engineering: a system/text does not get better by adding more to it, but when you can't remove anything from it anymore without changing your intent/its purpose.

Petzold could have done with a shorter post ;)