Because the official spec says Wave 2, other websites write 4x4 MIMO.
So, it's confirmed that it can go up to 2x2 160MHz 1733Mbps. I'm using Asus RT-AC88U, which only has 80MHz for 5G band, which explains why I get 866Mbps. For 2.4G, Im getting 2x2 40Mhz, tricking me to think it's 4x4 20Mhz...