I finally cracked it! For the longest time I just avoided EA games on my Deck because it bothered me that I could not play them offline with the EA App Launcher. But you can!!!, and it's super easy.

Step 1) While connected to the internet enter Steam Offline Mode (but stay connected to the internet).

Step 2) Open an EA App game. It will ask you for your login information.

Step 3) Exit the game from within the game (don't force a game quit with the Steam UI).

Step 4) Turn your WIFI off (either directly or via Airplane mode) and start the same EA Game from step 2.

Step 5) The EA App will now ask if you wish to enter Offline Mode, tap the screen or use STEAM+Right Touch Pad to select "Go Offline".

Step 6) It will take you to a game selection window where it won't take inputs, just wait about 10-30 seconds and it will prompt you to select "Launch Game".

Step 7) Enjoy Battlefront II on an airplane or a submarine .