In the U.S., (I see you use cm. Metric is too difficult for us Americans,) there are several types of stairs that “bend” around corners, etc. We call them 1) spiral stairs, 2) winding stairs (your stair design), 3) curved stairs.
One of the most important requirement in the Code is that the stairs must be uniform. That means uniform height, depth, depth at ...
That makes sense the installation video from Oatey says "wateproof drywall or cement board" but your kerdi board is waterproof too, so it's equivalent.
Be sure not to screw through the shower liner--if you need to screw low on the Kerdi board put a dwang (cross-block) in above the liner and screw to that.