If all is true, you can install Windows on the Steam Deck so I think it could be possible to use trainers on the platform, but we won't know until the hardware is released
I believe it is; This is from the Steam Deck FAQ:
Do I need a Steam account to use Steam Deck?
The default Steam Deck experience requires a Steam account (it's free!). Games are purchased and downloaded using the Steam Store. That said, Steam Deck is a PC so you can install third party software and operating systems.