But the software would probably still run on Linux even if you didn’t run it on a deck. All of that is control mapping and nothing the deck provides is specifically unique. The game would be developed on not-a-Steam-Deck most likely.
Gameboy games aren't typically developed on Gameboy (though NES games used to be!) and most consoles will run on linux via emulation or ("simulation" in the case of Playdate).
The steam deck has some unique features. And regardless, it's just about sparking creativity. But I am curious if anyone has gotten Aperture Desk Job (Steam Deck exclusive) to run on another device.
A Steam Deck game jam would not be simply a Linux game jam. The form factor and capabilities of the device could have creative uses. The device includes unusual rear buttons (R4, R5, L4, L5), touch-sensitive joysticks, dual trackpads which are also clickable, etc. -- it's not really fair to say that targetting a game for this piece of hardware is simply targetting Linux, just because the device runs Linux.