The parent means, this company can't just sell "a Gameboy plus a flash cart" as their own commercial product. Which is why they created a custom product instead.
You, as an end user, certainly can "just" use a gameboy + a flash cart, or an iOS or Android device, or even (and I would suggest this) code games using an old console emulator as your runtime and don't worry so much about hardware. (Emulators for those same consoles also exist for iOS/Android, after all.)
But this company has other priorities, and so made this.
I think you actually can since the software runs on the bare metal (there's no OS or anything) and it uses standard ish chips. There have been plenty of Gameboy and NES clones in the past that ran retail games.
