Remember, concrete products are at their rated strength after 28-days. The required shear strength of the bond to Kerdi is 50psi, but it tests out at 75psi. Real uses don't have huge cantelevered loads, only shear. On a tiled wall, any local load is shared by the adjacent tile since they are tied together with grout on a vertical load (shear), but a much less tenacious situation in tension.
I haven't received any official response from anyone yet, but it appears that the primer should provide all implied requirements - it's laytex based to prevent moisture migration and it is compatible to concrete based bonding to anchor the thinset. We'll see what the makers really think.
Best practice is to not use drywall mud in an area that is going to be tiled, regardless of what is going over it, Kerdi, or just tile (say in a backsplash).