Both macOS and Windows have alternative window managers available, although macOS does need to be mutilated somewhat heavily to make it happen.

Weirdly, the fact that the Windows and MacOS versions will be paid makes me more optimistic.

Customizing at least the Windows window manager isn't for the faint of heart and its architecture doesn't have a lot in common with Linux so such an effort is very far from a straightforward port, and as a result most Linux desktop software and especially stuff that deeply integrates with the desktop environment is basically never ported or the port is incomplete, buggy, short lived, etc.

KDE4 was supposed to fully support Windows and 15+ years later I'm fairly sure that dream is dead.

125 fps should actually be a huge red flag, not that the video FPS is the be-all-end-all of what the render FPS actually was anyways, as that's extremely unlikely to be their refresh rate. Since the other video has a different (but equally odd) refresh rate, we know it isn't which also means we know there would at least be judder.

This all strongly hints to the videos being variable frame rate encoded. A quick dump of the timestamps with ffprobe and then a quick transform to the deltas seems to agree with this https://pastebin.com/raw/PbbNGBVy

The most common frametime is 0.006945, which aligns with a 144 Hz target refresh rate. This makes sense as 144 Hz makes perfect sense as their monitor's refresh rate. Ignoring timestamp rounding differences, these are the inconsistent frametime buckets:

   0.006945, 0.01389, 0.020836, 0.027782, 0.034726, 0.041672, 0.048617, 0.062508, 0.076399, 0.097235, 0.10418, 0.118071, 0.145852, 0.166689, 0.229196, 0.256978, 0.29865, 0.354213, 0.395886, 0.513957, 0.770935

Watching a VFR recording of a 144 Hz desktop on a 120 Hz display may still seem smooth to you (after all, movies are 24 FPS and most online videos only 60 FPS) but it does not preclude frame targets being missed, as the data shows.

VFR video is relatively uncommon as well, so I wonder if that's why people are reporting so many performance issues viewing the video with different setups. I.e. between all of the reports of stuttering, it's probably both the video itself and the devices trying to play the oddly encoded video.

Yeah, it's outstandingly smooth for a web video.