According to the FCC Filing for the X on 5/24/13 , there is one LTE antenna to support 780 Mhz and 1730 Mhz. I am not saying it cant support MIMO, but unless they changed the antenna design since the FCC filing, it could be the dual band is being confused for dual antenna. The filing shows three radiating antennas LTE, GSM/CDMA, and WiFi/ Bluetooth(GPS antenna does not radiate so it is not shown). its in exhibit 11 part 1.
By the way the Droid Ultra and family devices have a similar antenna arrangement and also are listed as 780/ 1730 Mhz support