With any Kerdi installation or Kerdi DS installation you need a smooth clean surface. On this last shower build my pre-slope was a hair out so I chipped away some of the material.
This is not a typical mud bed installation so do not try this with mortar mix or 4-1 mix.
The video shows me striking the edge of the pre-slope where the cement meets the drains (ACO FLANGED) edge. Here the pre-slope is less than 3/16" and look at the strength. I took me a few minutes of striking and grinding to get the pre-slope the way I wanted it. For more info on this ACO flanged drain install on this custom 65' yacht check out this thread. Installing Showers in Yachts