Brian,
The issue of HDR on streaming video sites as displayed on the Apple Pro XDR display is a complicated one, and I am by no means an expert, although I am struggling to learn as you are. I have the 2019 new Mac Pro with the AMD Radeon Pro Vega II 32 Gb video card and the Apple Pro XDR display with standard (not nano glass). In my computer room the display sits to the left of a large bay window. Fortunately, the window is situated such that reflections are not an issue. However, this is a rather bright environment for the display especially when viewing HDR content. In this environment, I find that the XDR display in any mode other than the default 1600 nit mode is dimmer than I would like and Apple does not allow you to change the brightness in any mode other than the 1600 nit default mode. Thus I use the Apple Pro XDR display for all purposes in the default 1600 nit mode.
With the above said, I do not believe the video which you referred to on Vimeo is in HDR on Vimeo. I did find the exact same video posted to YouTube and it definitely is in HDR on YouTube. I made certain to watch the video on YouTube in Chrome (since Safari does not allow any higher resolution than HD and you want to watch the video in 4K + HDR +WCG). When I watch the video in 4K with my XDR display set to the default 1600 nit setting in Chrome, I can readily see that it is displaying HDR with very bright highlights while still retaining good shadow detail. The highlights are very impressive even in the relatively bright environment of my display. I should point out that shooting HDR footage requires careful control of exposure, and in my opinion there are several shots in this video where the highlights are totally blown out due to over exposure or improper color grading of the video. Other shots, however, are truly excellent examples of the value of HDR with great highlights and rich blacks.
I honestly do not know whether the 16 in. MacBook Pro and your video card is capable of utilizing the full capabilities in terms of resolution, color gamut, and HDR so unfortunately I really cannot answer the question central to your post. Nonetheless, I hope this has been of some interest to you.
Tom
