Hey there, unfortunately it looks like the mods or bots may have nuked your linked post, so I can't check it out.
As for HDR on the Tab S8 Ultra, it seems to support HDR10+ per the Amazon and GSM Arena (who I usually go to for specs):
https://www.amazon.com/Samsung-Galaxy-AMOLED-Screen-Graphite/dp/B09WQM6H4N
https://www.gsmarena.com/samsung_galaxy_tab_s8_ultra-11274.php
In my testing I did not notice the brightness issue on my Tab S7 FE (which has a more basic LCD display with no HDR support), so I assumed it was HDR related and not the android app itself.
I haven't watched much on Amazon lately so I can't say if it has gotten any better. I've been thinking about starting The Boys, which likely has some night scenes, so I'll report back if I notice any improvement.