But any competently inserted intentional backdoor is going to be indistinguishable from a mistake.

If Cisco had some SecretFBIChinaBackdoor() function somewhere the backlash would be way way worse (or at least an unknown). Whereas at this point it's abundantly clear that serious "non intentional" security vulnerabilities in networking hardware basically go ignored by the market.

Yes, that is the most commonly used to describe an “undocumented” access credential, regardless of why. This has been used for credentials that were added and forgotten, credentials that were added to permit unauthorized access later, and credentials that were added to permit authorized repairs more readily. These credentials were included in an “undocumented” (or “unpublished” might be more precise) manner and can be used to bypass security, so “backdoor” is correct.

This of course says nothing about whether its inclusion was due to intent, incompetence, and/or malice. (If the private key includes “Comment: hack the planet” then yeah it’s malice :)