The remarkable is more open than usual and that's great. That being said, if your priorities are openness, you should look at Onyx or Likebook devices, they run android and you can root them. That alone is leagues above anything in the Remarkable.
They are cheaper with better screens and more powerful processors, too.
As a practical matter, for developing, a rooted Android device is going to be better for developing as everything's standard and easy to work with.
As for GPL compliance, that really only means anything for the kernel. For the rest of the software, they can easily be closed sourced and not provided. Not that they necessarily wouldn't be, but I'd still rather have an Android device with an unlocked bootloader and no kernel sources.
They are cheaper with better screens and more powerful processors, too.