Author of Ripcord. You’re wrong, and creating and using software compatible with services is fair.
It's by societal consensus that this can be made fair, and to convince people that it is fair, there needs to be a logical, indisputable argument that it's fair. Otherwise, the service providers can always just hide behind the counter-argument of being in control and TOS etc.
This seems to make it pretty clear they don't want you to create 3rd-party clients for their service.
"You may not create derivative works based upon any of our services"