I think you really should qualify this for the sake of all of us amateur writers out there. I would've said "people severely, severely overestimate the amount you can make self-publishing."
Most of us don't have a book idea worth writing about or are really bad at writing, or both. It's also really hard for a lot of us to realize which group we're in.
You say "I raked in $TONS for a $40 non-fiction book that helps folks with business development" but I think the rest of us should recognize those details when we see your numbers. If I want to write a book about the subtle great things about unit tests, python, or the great American novel, I should tread with caution. My boring story will probably not make $TONS.
> very best authors they published would sell 10,000 books. Their cut was something like $2/book, they'd pull in $20k.
This is a more interesting part of your post, and worth highlighting. Self-publishing can net you a LOT more than working with a publisher. But for sure you've got to start with an idea that people think is worth buying.