I see. I would suggest looking up other environment variables the Steam Deck sets on boot, but since this doesn’t look to be the case, my second suggestion would be hardware spoofing.

But even then, it just doesn’t make any sense. Why support only the Steam Deck specifically when it’s well known that Valve is updating SteamOS to make it work with other handheld devices like the Legion Go? Assuming they put some kind of hardware checking in the game initialization phase, why go through all this effort just to lose potential players?

Might be unrelated, but Marvel Rivals has a similar problem. If you just download it and play straight away, it won’t open; you have to set an initialization flag SteamDeck=1 to make it open (source)

Maybe it’s the same situation and this flag solves the problem, or maybe it’s something similar