I think the Deck has the advantage simply because it’s that much easier to develop on and for a PC than a console. At least I would think so, you would have access to more tools and libraries and debuggers than you would with a dedicated console.
Since the Steam Deck is still fundamentally “just a PC” [1], it has the advantage not really requiring specific “ports” of the games? At least that’s how I think of it, again I don’t work in the games industry.
Of course making stuff “Deck Compatible” might require some specific work to make sure stuff can run on it at a playable framerate, so it’s not completely free, but it does get the advantage of inheriting pretty much every last-gen PC game automatically.
[1] in the sense that it’s running a fairly standard Linux kernel on x64 hardware, with typical desktop Linux features available.