This. I was reading about some of the ugly hacks Valve has had to get around to use 2.1 on the steam machine. They (HDMI consortium, whatever its called) won't let you use 2.1 if your video drivers are FOSS. Since SM has open drivers for the AMD card it's leading to subobtimal video output at certain resolution/framerate combos (4K@120fps? Something like that), and they can't legally advertise support for HDMI2.1.