MU-MIMO (Multiple-User Multiple In Multiple Out) does [math] to assist in directing signal to multiple clients at once via multipathing, which this reflector would fuck with the math of in (I think) a detrimental way. Regardless of its impact on that technology, higher-end wifi phy rates (the negotiated modulation rate between 2 stations, i.e. the wifi access point/router and your phone) would get shredded by having a reflector bouncing signal between the multiple antennas, forcing clients into artificially lower speeds for a [potential] marginal boost to gain.

This stopped being a helpful thing to do somewhere around the transition between wifi 4 and 5 (802.11n --> 802.11ac)