I’ll go a step further. I agree going through all of the trouble writing and promoting a book for only a $100K seems like a lot of work. Especially when you consider the risk adjusted return.
Anyone that smart could make more than that - guaranteed just by consulting.
I’m most familiar with the Apple related ecosystem, but there you see people with a following making a money via optional subscriptions (Accidental Tech Podcast,Upgrade), subscription only podcasts (Dithering by John Gruber and Ben Thompson), or daily newsletters (Ben Thompson).
If he is not doing just for the money, then go for it. Another example is John Siracusa of ATP. He wrote large reviews of each version of OS X for the first 10 years that literally took months, did much more than his contract required and admitted that if he actually accounted for every hour he worked on it, it paid much less than minimum wage.
One more mention is Horace Deidu. He got his start as an analyst, started podcasting and now hosts paid workshops and conferences.