ur right, even though they are both gcn based, the video decode asic is actually different. while my vega igpu has already been upgraded to video core next asic for video decode, polaris is on its predecessor unified video decoder. if i could test it out with my 580 i woudnt have made that post, but sadly it has moved on

based on the compatibility chart
here, ur 570 does not support hw accelerated vp9 decode so that is why it is sw decode, where ur cpu is not fast enough for smooth playback.
another bro kena same problem a couple of years back:
https://forums.hardwarezone.com.sg/threads/4k-video-playing-problem.6249680/