This guide shows how to make the game stretch to the 800p screen of the Steam Deck.
How to Stretch to the Fullscreen
On the library entry for the game click on the gear icon and select Properties
Under General scroll to the botton section LAUNCH OPTIONS and input the text:
The setting will save automatically when you leave this menu.
Open the game and now the Scaling Options will work:
Default 16:9 scaling, notice the black bars on top and bottom:
Stretched mode which removes the back bars: