I wonder if they are using per device wifi queues. You still have round-robin issues once you have more active devices than mu-mimo supports or two devices are in the same direction. mu-mimo uses beam-forming to break up broadcast domains and does not work if the two beams overlap.
The reason per device network queues are important is because wifi is only so fast because it can send bursts of packets, but when you have two or more devices causing interleaving packets, burst no longer works. eg
Say you have one device and below are the packets
Device1
Device1
Device1
You can batch up those three packets and send them at the same time
But say you have two devices
Device1
Device2
Device1
Device2
Device1
Device2
You can no longer batch the packets. Per device queues would instead do
Queue1
Device1
Device1
Device1
Queue2
Device2
Device2
Device2
And round-robin the queues instead of the packets. Some researchers have tested this out and it makes a HUGE difference in throughput and latency. Of course no known routers do this yet, unless you install custom firmware.