I assume by that they mean that Source 2 will be running on Windows Linux and Mac, but also the next gen consoles, so the input device is up to the game.
...since the third is obviously going to be something based around input (controller?)
Edit: you really think the third and biggest announcement is what input devices it will allow?
"Today, we reveal: You can plugin a mouse, a keyboard, a gamecontroller, and a joystick! Thanks for your attention, that was all. "
