VP9 hardware decoding on Big Sur

As of Big sur, it supports VP9 hardware decoding on Safari (which enables 4k playing on YouTube)

I tried to test if it supports well on Hackintosh with config iMacPro1,1 and AMD 5700 as discrete graphic card but failed to work, still software decoding used causing insufficient power consumption and high cpu usage.

I tested playing 4k youtube video on BigSur on Macbook Pro 2019 and it works great with VP9 hardware decoding to achieve less than 10% CPU utilization. However, i noticed that it utilizes integrated graphic UHD 630 instead of discrete graphic card as shown on activity monitor.

I just wonder if VP9 hardware decoding currently supported on BigSur is only for integrated graphic card only? It is welcome to share with any idea for people who has ever looked into. I would appreciate that!