6k is what the book brought in but the author has only been paid 1.5k. And given that the author paid his friends to review it, this project seems like it's pretty close to netting zero.
But you gotta start small. The hope is that one could continue to do this side hustle, until such times as this hustle gets enough income to allow one to comfortably quit the FAANG job without worry.
Of course, not many succeed in this endeavour - it has to be the case, otherwise, nobody would be working at FAANGs today! However, it's not worthless to try.
You could say that of any number of side hustles that are <1% likely to approach $350K/year. Write a book or don't write a book. I've done a few and it's been a good, if not especially directly profitable, experience (at least in retrospect).
I can make more money in sometimes ghostwriting content marketing content (~$1K/day). As with many things, you basically need to know people to get that sort of work--though that's often true of going through a publisher as well.
How do you find gigs in which you're paid 1k per day to ghostwrite marketing content? I've worked in this space and that seems unusually high. Honestly curious.
I should clarify that the author said he had technical people he knew review it during the production phase. He was not talking about paying money to get positive reviews on the Amazon or that sort of thing.
Of course, not many succeed in this endeavour - it has to be the case, otherwise, nobody would be working at FAANGs today! However, it's not worthless to try.