@n_b,
It appears as though you are correct. The AX75 and AXE75 both do not have support for MU-MIMO. This is why the speed of the router is also slightly lower than the other routers. For MU-MIMO operation, you would have to look at the next step up in routers, like the AX/E 90 or 95, or look into one of our gaming routers, or the new AXE300.
We were provided an update from our engineers on the topic. It appears as though when the functionality of MU-MIMO is advertised on individual products, it is referencing the NEWEST versions of MU-MIMO. For the AX75 and AXE75, the MU-MIMO application is present, just based on the older version that is not necessarily optimized for Wi-Fi 6 connections. As a result, it is not listed as an advertised feature; however, these devices do support 2x2 MU-MIMO rather than 4x4 MU-MIMO, meaning that they can only support two simultaneous upload and download streams, rather than the 4 streams possible when in a 4x4 configuration.