Yeah it's basically like a launch option that will configure it in a certain way. You can in fact with a launch option tell the game to ignore this, and play split screen on Deck. I have seen people doing it but I doubt it runs well but I guess that's the beauty of PC/Deck gaming that you can do whatever you want to and make up your own mind.

The default profile for Steam Deck disables split screen. You can enable it but it will run like shit. It running at ~30 fps with the default profile means it can be verified.