Earlier quoted context omitted.
You believe every tech oriented person already knows everything there is in the book?
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.
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).