Self-help non-fiction books killed themselves by focusing on entertainment, in the form of amusing anecdotes, rather than substance. Most self-help books could be reduced to a 3-by-5 card without losing any of the core information.

The article gives anecdotal evidence that combining personal stories with advice in a book is more effective than delivering distilled information.

Conversely: the self-help nonfiction book existed because it was the only practical way to monetize "good advice" or "good ideas" at scale. Now you can do a podcast or a youtube series and try to make money from advertising/affiliates/etc, but for a very long time, "buy my book" was the only game in town

The author even mentions this. Why watch a 20-minute video when you can just scrub to the 40 seconds you need?

A lot of self-help books fall into this category. But if you go to a publisher and say that you're going to publish a 20-page book, they're going to laugh you out of the room.