If they claim "here's the API you can use to control network access" but can then put arbitrary apps to work around that, that's the definition of backdoor access to the device.

A "back door" is any kind of mechanism that was added by the vendor to circumvent security mechanisms.

Back doors are typically not disclosed to the user, and can't be turned off. So for example an automatic software update mechanism isn't a back door, as the user is aware of it and can typically turn it off if they are concerned about security.

An undisclosed mechanism that allows Apple apps to circumvent firewalls does very much fit the description of a back door.

Intent doesn't matter with regards to back doors. Most back doors are not made with malicious intent, or at least the vendors usually claim that they only had good intentions for the back door. (Eg. see the recent reports where a router manufacturer had a secret password that they claimed was only used for software updates)

The danger about back doors is that malicious software can use the back doors to circumvent the security measures, just like Patrick Wardle demonstrated that it was possible to use Apple's content filter exclusion to circumvent firewalls.