Incorrect. "Key" in relation to "backdoor" and "security mechanism" does not always mean "encryption key." You are confused because there are more than one security measures at work. This removes the key to one of them, completely circumvents it instead of going through that "door," so to speak. That is a backdoor. It's a very important security measure too, as evidenced by the fact that it's the only one they need to have removed.
It is UNDENIABLY a back-door around the self-destruct/lockout mechanism designed to prevent brute-force attacks. It is undeniable that this could be used on anyone's iOS device if it could be used on these particular phones. They are literally asking Apple to open the locked doors to millions of innocent customers. It's not OK. That is like having a general warrant. Even if they leave control of it in Apple's hands, Apple will have been forced to circumvent the very security they worked hard to create and it will affect the potential business success they had gone through the effort to obtain.
Disabling a feature that is meant to keep someone from guessing the password, allowing them to crack the password by bruteforce, is a backdoor. It is. Period.
The commonly understood definition of a backdoor is:
A backdoor is a method, often secret, of bypassing normal authentication in a product, computer system, cryptosystem or algorithm etc. Backdoors are often used for securing unauthorized remote access to a computer, or obtaining access to plaintext in cryptographic systems.
While I agree the FBI wants software to obtain unauthorized access to the plaintext data stored on the phone the reported method in no way bypasses normal authentication. In fact the FBI is relying on the authentication system to authorize them to access the decryption key.
If the software was a backdoor there would be no need to brute force the password.
If your interpretation of a software backdoor was correct every system that relied on a password for authentication has a backdoor as anyone can attempt, (at least a limited number of times), to guess the password. That's not how the term is commonly used.
At any rate we're arguing semantics. I've stated my position so I'm going to leave it at that.