I very much doubt that that's accurate, especially for the CG, and
especially for HFR. It increases your overhead; HFR means there's four times as many frames, UHD means every frame is four times the size, so that's 16 times the load on your computers rendering and editing and color-correcting when you're just going to throw it all away. And they'd have to have thrown it away, because if they made a UHD master, the show would be streaming in UHD, just like SNW, and if there was an HFR master, someone in a position of authority would be drunk, because people hate watching high frame-rate footage except for weird gamer-types.
The YouTuber's rationale for thinking this seems to be that it's within the technical capabilities of the cameras they used to shoot UHD and HFR, but that doesn't mean they were actually used that way. Shooting in UHD for an HD final did probably happen, but only so they could adjust framing after the fact without losing quality by cropping in a little (and it would've been downconverted to 2K fairly early in the pipeline, so they'd need to remaster it to release it in true 4K and not upscaled), but shooting at more that 24 frames per second for a 24 FPS deliverable would be stupid and wasteful (with certain exceptions, like shots with slow- or fast-motion effects). It'd just make extra work somewhere else for no gain.
In fact, I can
prove it was shot at 24 FPS, thanks the the miracle of
dog-mouthing: In this
behind-the-scenes photo from
this tweet, in the upper-left corner of the camera viewfinder, you can see it's recording at 23.976 frames per second. Case closed.
All that being said, a 4K Blu-Ray release for PIC would be worth it for the HDR and higher bitrate. Especially the HDR, and especially in the third season.