It is not. I was able to do 4k120 with mo123 image. People here just reply without even reading the specsheet. It can absolutely output 4k120.
Get it from here [ROM] Rock5A/B AndroidTV 12 by mo123 - ROCK 5 Series - Radxa Community
You need to PM mo123 to get the password. I get 4k120 at 4:0:0 but depending on your setup you can do better. I haven’t tried HDR, I’m assuming it’s a no go.