You wouldn’t use fasteners in the joint itself, and you’re exactly right that they would pull right out. The screws are used to fasten through the face of the wedi board to wooden studs behind it to provide strength, and then the penetrations and joints are all sealed with the wedi sealant. The board itself is waterproof, you’re only sealing the joints/penetrations. I believe that small shower niches with 1/2” board are sturdy enough that they don’t even need wooden framing and just rely on the board+sealant, but am not sure on that.