Not true. To say something has a "back door" is to imply security was deliberately compromised by the manufacturer, whereas a vulnerability is a security weakness in the the architecture or implementation.
I get what you're saying but it would have made more sense had Apple not added the Secure Enclave to newer iPhones. I.e. they tried to make it more secure, so I have to conclude that they had tried their best with the older versions, too.
A simple flow chart or state diagram would show the vulnerability. I'm not saying it was intentional, but I believe the backdoor label can be applied to accidental entrances as well.
The bottom line is that Apple produced a device whose security features could be circumvented.