quoted

What's the point of this indirection other than another way of avoiding
empty node 0?

Honestly, I do not have any idea. I've traced it down to
Author: Andi Kleen [off-list ref]
Date:   Tue Jan 11 15:35:48 2005 -0800

I don't remember all the details, and I can't even find the commit
(is it in linux-historic?).

But AFAIK there's no guarantee PXMs are small and continuous, so it
seemed better to have a clean zero based space.

Back then we had a lot of problems with buggy SRAT tables in BIOS,
so we really tried to avoid trusting the BIOS as much as possible.

-Andi