I wonder why the TTMAC specifies cement board in steam shower builds? Perhaps they feel that backer boards should not dissolve with heat and steam. In the TTMAC Specification Guiodelines there is only one backer board listed - It's Cement Board.
If you want Kerdi Board then make it. Use cement board and then cover it with Kerdi. You want a better board. Use cement board and cover it with Kerdi DS!
The facing on Kerdi Board is waterproof not vapour proof. Think about that - do you want a backer board that relies on cardboard to have that cardboard get we with vapour. Take a piece of cardboard - hold it over a boiling pot of water. What happens to the cardboard, I'll tell you - it weakens and then drops or bends down.