Side note: HDMI Forum pulled a USB and fucked their standard so forum members didn't have to upgrade their hardware. In the same way that USB 3.0 became "USB 3.1 Type-1" you can take HDMI 2.0 and essentially market it as HDMI 2.1 directly - it doesn't have to support 120 hz / 48gbps mode, it doesn't have to support VRR, etc. If it passes the HDMI 2.1 compliance testing, HDMI 2.0 hardware can be marketed as HDMI 2.1 without change.
So it could be HDMI 2.1 compliant, and still not support 48gbps mode.
(Which isn't quite the same thing as not supporting "120 hz" - it could probably do 120 hz over displayport/thunderbolt, or at a lower resolution, or with chroma subsampling - just not 4K120 4:4:4 because it doesn't have enough bandwidth)
So it could be HDMI 2.1 compliant, and still not support 48gbps mode.
(Which isn't quite the same thing as not supporting "120 hz" - it could probably do 120 hz over displayport/thunderbolt, or at a lower resolution, or with chroma subsampling - just not 4K120 4:4:4 because it doesn't have enough bandwidth)
https://www.youtube.com/watch?v=qo9Y7AMPn00