"Bug" implies a mistake/oversight where the additional functionality was known to noone, and then discovered. This functionality was deliberately created, thus it's a "backdoor".

Based on what seemingly passes for "accepted practice" in the mobile world (download QPST for tons of fun!), the only sane way to have a trustable mobile device is with a separate cell-modem and a well-defined interface.

I mean, maybe I'm confused, but this sounds like they closed one method (among others) that could potentially be used to create a backdoor by, I guess, a carrier or OEM.

There's no evidence that anyone's phone was open to remote exploit at any time.

It seems as if you're using a weird definition - "backdoor" just implies that the functionality exists, not that it has necessarily been utilized.