But surely that relies on a bunch of “UWP” and Windows 8 APIs? The screenshots I managed to find make it look very much like it relies on the Metro UWP garbage.
Windows 8 shipped with build 8400 (apparently), so I assume this PDF reader is pre-UWP rewrite from one of the early Windows “8” builds, that were mostly Windows 7.
As far as I could tell/recall the Windows 8 Reader app was always a thin wrapper around Edge (Spartan) (which is also when it broke, when Edge switched to Chromium). I suppose it is possible it worked with Trident (IE11), too, or the "Service Pack" includes some version of Edge (Spartan), which would have been all the hard work on that, though that isn't listed.
They extracted the .exe from the appx.