Suffice to say, if they couldn't figure out how to make it work in 64-bit, they're not going to be putting any effort into making it work in UWP and on ARM!
It will run on 64-bit OSes as-is, because they support 32-bit programs. But if you check out Raymond's response to a comment, he explains that at the time, 32-bit apps weren't supported in 64-bit Windows until much later. And even once it was possible, it wasn't considered proper to ship an OS component that wasn't actually 64-bit. (Note that while everyone's still been using 32-bit Internet Explorer for the most part, 64-bit IE has always been included on 64-bit Windows OSes.)
IIRC Windows XP 64-bit was supposed to be a pure 64-bit OS - no compatibility layer for 32-bit apps. Which is most of the reason it didn't get some great adoption.
Later versions of 64-bit Windows had that compatibility layer.
Windows XP 64 Bit Edition was 64-bit only and ran on Intel Itanium processors. Windows XP Professional 64 Bit Edition ran on AMD64 architecture and included 32-bit compatibility with x86 Windows, but not 16-bit compatibility with DOS. It provided a preview of the broken drivers that were part of the Vista rollout.
64-bit XP ran 32 bit apps just fine. A little more than a decade ago I built a machine with it, and wound up giving it to my very much non techie parents who had no issues with it.
I think the only thing I had trouble with on that machine was finding drivers for a particular printer.
