Is 802.15.4 supported on the cc2650 LaunchPad XL?

Yes indeed, there is no driver for the cc26x0_cc13x0 radio yet.

There is a driver in the works for the radio on cc26x2_cc13x2, maybe it can serve as a start? (Although mind you that the new chip is a 802.15.4-2012g device, so it will have some differnces).