The idea is to make the subfloor flat. By cutting the plywood that way the oposite is achieved the plywood will be more flexible and it will fillow the existing surface on which the subfloor is installed on https://youtu.be/7l6ts1YwnfU
The video shows the installer adding thickness under the new flooring material so that the new floor will be level with the existing tile that was 1.5 inches taller (I got that from the video description) . The video was not talking about leveling a wavy floor. The installer was using 3/4" plywood as an underlayment, and as you pointed out, by cutting the kerfs into the plywood, it will follow the waviness (if any) that already exists in the subfloor.