Even more basic...the tile are not to be considered a waterproof layer...what's underneath it must be water resistant and not degrade if it gets wet as no matter what tile you use, some moisture will get behind it. The cbu is not waterproof, but it is water resistant in that it doesn't degrade when it gets wet. Drywall will fall apart and start to support mold growth when it gets wet.
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters),
Mapei Training 2014, Laticrete Workshop 2014, and Longtime Forum Participant.