I beleive they left off the 400mm f/2.8L IS becuse that lens could already focus on the 1DX with the 2X extender prior to the firmware update, because it's max. aperture is only f/5.6.
list 300/2.8L IS, 300/2.8L IS II, and 400/2.8L IS, as well as a whole bunch of other f/2.8 primes and zooms, all of which could AF at f/5.6 with cross-type points prior to the v1.1.1 firmware. They also list a bunch of f/2.8 and f/4 lenses on the 1.4x list, and on that list, the 400/2.8L IS II is
included. They just don't list the 400/2.8L IS II on the 2x list.