Isn't there a Retroarch core for Doliphin? And Retroarch is on Steam. Maybe the Dolphin core is not available on the Steam version though.
Anyway, thankfully if you are tech savvy enough to get Dolphin to work, you are more than likely also capable of side loading it on the Steamdeck.
Serious concern and asshole move? Yes. Gpl violation? Not sure. You could argue you are not restricted to do whatever you want with the code you receive with a subscription. But if you share the code, they don't want you as a customer anymore and won't give you new code. I don't know if the GPL allows that.