The author wrote many technology introduction books over a period of six years. They probably have a limited lifespan. I think the comment is somewhat fair if overly snippish - “How I earn $27k per year writing books in my spare time” would have been more informative and accurate than calling this passive income. If the author stops writing books, will they earn another 160k over the next six years?

I still think it’s a cool and fascinating article - congrats to the author on their success!