Where did you got this idea from what I wrote (in fact, from my direct quote of Brooks' law)?
>That's an insane idea (it does lead to the conclusion that the optimal team is zero sized)
That adding people to a late project makes it later doesn't "lead to the conclusion" that "optimal team size is zero".
That's what's actually insane (one common form of insanity is following a logic to extremes without caring for nuance and limits).
Just that more people is more overhead (e.g. managerial and communication and agreement overhead), more people later equals more time to bring them up to speed and hand-hold them until they're ready plus the added overhead (related with more people) even when they're ready.
Brook's law doesn't say you should never put more people on a late project. It does say that you should not realistically expect the project to finish at (or sooner) than the initial estimate because of you adding more people.
In other words, for a late project with X persons working and M estimated months to completion. The real completion with X persons might be MR months, and with more persons X' it could get to MP. Adding extra programmers won't (per Brook's law and based on typical observations) ever help it reach M.
Sometimes adding more persons will make things worse, where MP > MR, other times they can help finish faster than the "actual" (not the initially estimated) finish date, so that MP < MR.
So it might still be advisable to add extra people -- it just wont (per Brooks law) get you to finish in M, and in some cases might even get you further from MR.