I'm not sure if reading the book would help. If the manager has a technical background (e.g., they have worked as developers before) then they already know the main point of the book. If the manager does not have a tech background, then there's little that the book can do for them.
If the only important information in the book was its main point, it wouldn't have needed to be a book. It could have been a leaflet. Or a bumper sticker - those can be very catchy.
It's worth reading the book for all the other words it contains.
No, but most will at some point in their career hear about it in comment sections on HN and the like, or in many different forms hear the adage that adding more people to a project makes it later.
Many developers don't spend time on hacker news or even any of the similar forums. To be stereotypical, HN readers seem more likely to be young, working for FAANG in Silicon Valley, and not a middle aged sharepoint integration developer living in the Midwest (although obviously they are also on this site).
I've had developers who treated it like a job, did it, went home, and weren't interested in the Mythical Man Month (but intuitively knew some of these principles).
