Would HDR10 and Dolby compliance have worked well on a 828p display? Realistically, only 720p 60 FPS HDR content would've occurred, right?
HDR is more about the dynamic range.
At the risk of greatly simplifying, HDR is letting the supported contents (which are most commonly HDR10 or Dolby Vision formats) utilize all or most of the display's "luminance" range (or brightness level). The end result is that you will see contents that can utilize both extremely dark and very bright details.
Hard to explain with words, but imagine contents where some portions are set to the lowest brightness setting and some portions set to the highest brightness setting. Add more color and advanced color management to the mix, you get the gist.
If you have iPhone with OLED screen (X, XS, XS Max),
this YouTube channel has a ton of demos.
iPhone XR supports and plays HDR contents, but without the high dynamic range. So it's effectively not HDR.