The point was that not every game was confirmed to work. For a PC game to work on the Steamdeck, it needs to meet two criteria:
Work on Linux, either natively or through Proton.
Have controller support and/or be playable with a touchscreen.
Not every PC game meets this criteria. Some games still don’t play well in a cross-OS runtime environment like Proton or WINE. Others are designed specifically for mouse and keyboard, or keyboard alone.
One game I can definitely say is not Steamdeck compatible is SimCity 4. The UI doesn’t really work with touch screens well, the game has no native controller support, and it originally released with SecuROM so a physical copy won’t even work on modern Windows, let alone Linux.