I think the argument here is that stealing a private key isn't fundamentally more difficult than stealing this iOS backdoor (were Apple to create it). Under this model, Apple's refusal to create a backdoor is in some sense an appeal to security through obscurity because it implies that engineering knowledge is the main obstacle to adversaries, whereas the true obstacle is the ability to steal protected information from Apple (the key or the backdoor). If the key and backdoor were equivalently useful, the existence of a backdoor wouldn't impact the safety of customers because the key already exists.

Of course, whether stealing a backdoor is actually as hard as stealing a key is a legitimate question, but (I thought) Apple had the option to unlock the phone in-house, which would at least keep the backdoor out of FBI hands (the legal precedent, however, could still pose a real threat to user security).