I don't see anything on wikipedia saying it had to be x86, just that they're PCs with a small form factor.
I don't know why you're getting so hung up on the definition though, the term "UMPC" was literally invented as a marketing gimmick: someone said "hey, this tiny laptop thing, what can we call it to distinguish it from other stuff? Ultra-Mobile-PC sounds good to me, anyone disagree? No? SHIP IT!"
Is it a PC? By the true definition of the word. Is it mobile? That's obvious. Is it ULTRA mobile? Well, it's more mobile than a laptop, or even some of the other things already classified as UMPCs, so I guess it is.
Pandora is UMPC as far as I'm concerned.
I wasn't getting hung up on anything, I just thought they were different classes/types of devices. Palmtops, I thought, were basically tiny pocketable laptops, and UMPCs were bigger, more powerful and not as portable. It doesn't matter, I just viewed them as different things, but I was the only person making a distinction, I guess. That was probably due to how I wasn't very interested in what I was seeing that was called UMPC, but still very interested in palmtops.
As you said, UMPC is just a marketing term, by Microsoft and Intel if I understand the intro from Wikipedia correctly. It is that, and that they were designed to run Windows XP/Vista/7, that led me to say the x86 thing. Additionally, I didn't see any that weren't, but I didn't dig too deep. Since palmtop wasn't an official industry designation (except maybe by HP, but it is used for others as well), it could also be used for devices not using x86 CPUs. The "See also" section
here is pretty interesting.
To answer some of your questions, by how the term PC is used now, UMPCs were/are, the Pyra is debatable (I say no, since it has no ties to the original IBM PC and compatibles, however since it does offer most of the functionality of a desktop OS, perhaps). Ultra mobile, yes, at the time they were introduced UMPCs were, and the Pyra is even more portable than some of those, as are smartphones (and I think Pyra is more portable than some phablets, tablets and such). For most of us, either term will probably work, as does "a laptop that fits in your pocket".
@EvilDragon what can we do to help you make sure the Pyra goes into production?