The Denon + Marantz receivers have reportedly patched their AVR's via firmware to run 4K / 120. However, it's only on the PS5, not the XBox Series X (incidentally the PS5 and Denon's both share the same HDMI chipset manufacturer which explains this).

The HDMI 2.1 standard for AVR's was only actually released for testing in December... so to release an AVR prior to that and say it's READY is either very confident or freaking idiotic from Sound United! I know Yamaha never said there's would be ready out of the box in order to let users know what to expect... however S.United were desperate to be first and scream from the top of their marketing mountain about it.