I didn't say that adding new people to a project never makes sense.
Adding new people to a project slows the project down at first, because the new people need to be educated. How long this takes, depends on the complexity of the project and the matter. If the pressure and the deadlines for development are ok with that, it's fine to hire new people.
So what do we know about LFS? Not so much, only Scawen knows ... but from just playing the game one can already say that the complexity of the matter (i.e. the phyics of racing and the development of a stable program for multiplayer access) to me seems quite great.
This would be an argument not to hire a new developer.
We also know that there is no pressure or deadline at all. This would be an argument to think about hiring a new developer.
Any further arguments can only be judged by Scawen. And when he says he wants to work like before, I can well understand, because it would change his daily life significantly. From independent developer to supervisor. Being forced to lead hours of discussions with the new people about why this and that has been implement this or that way and so on ... Progress on LFS would most likely stop for months!
But of course Scawen has proven that he asks other people for advice regarding the physics of racing and he also does his own research (on track). So he's not all alone ...