The first round of Broadcom-based 4x4 routers (AC3100, AC3150, AC5300, AC5300) shipped without working MU-MIMO. That was the reason for the "MU-MIMO ready".
Since then, various companies have issued firmware updates enabling MU-MIMO, with varying degrees of success. Our tests showed they barely worked, if at all.
It also turns out Broadcom's 4x4 radio chip has problems with MU-MIMO. So manfs quietly phased in a newer version along the way.
The status today is you need both the newer device and new firmware to allegedly have decently working MU-MIMO. The exact combination needed, however, varies from manf to manf and isn't disclosed.
The FCC Class 2 Permissive Change letter attached, shows the first change as:
". Updating Chip model to BCM4366 C0 (MU-MIMO) from BCM4366 B1 (SU-MIMO)."
All that being said, if you want working MU-MIMO, get a QCA based product (AC2600 class). You will also need at least two devices that have proper MU-MIMO support. That isn't guaranteed either, as you will see in an article I will publish soon.