People always righteously wave that book about as if it was some little known religious text of ultimate authority, without knowing the nature of the project in question.
It is true, there are a lot of really good points in that book. Any project or engineering manager should know it and I've known too many who didn't. I've seen the prophecies sadly come true too many times.
However, we have no idea how Microsoft achieved this; was it a fully staffed project with specific deadlines? Was it many separate projects (an OS could have many areas to examine), possibly scattered in time? A small working group? A long running initiative with domain experts pulled in as needed and no specific deadline? Was it initially small but then grew as different separate problem domains were discovered? A specific goal allocated within each OS project? Etc. etc.
Until we know how they ran the initiative (or initiatives) we can't say at all whether adding more people would be or were bad.