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.