I contacted their support about the openness of their device. This is their response in case others are interested:
Aug 28, 2020, 3:44 PM GMT+2
Hello there.
Users can gain root access to the device by using SSH, so the device is open for developing your own software.
The GPL and LGPL version 3 requires us to give users access to their own devices. It's part of the anti-tivoization clauses in the licenses.
A lot of our software is open-source. You'll find a lot of our open source code here: https://github.com/reMarkable/. If you're interested in developing for the reMarkable, a good place to start would be on here https://remarkable.engineering/deploy/.
Note that we do not currently provide any support for SSH related issues. Accessing the device and making changes through SSH is at the customers own risk.