Onboarding is a real thing but Brook's calculations are absurd by today's standards. 1 Month to onboard someone? We have developers committing code in their first week. Sometimes day 1 or 2.
Read McConnell's essay. It's v. good.
That doesn't mean they're fully on-boarded until 6 months in.
Brooks exact numbers might be come from a different time, but the principle holds unless you have an uncommonly decoupled set of problems.
If you're doing anything complex (which can just be: if your codebase is more than a year or so old) than the quality and velocity of code someone can contribute on day 1 is vastly lower than they can a month later.
I see the Mythical Man Month tossed around as an excuse for not trying sometimes - by people who haven't even tried to split up and spread out chunks of work in parallel - but onboarding cost is definitely real.
You gotta evaluate "can I get enough overall throughput at this point in time to make up for the total efficiency lost" and also the rest of the business priorities outside of this one project.