It allows anyone who knows the default SSH key pair to login as root. How is that not a backdoor?
Backdoor definition: "A backdoor is a method, often secret, of bypassing normal authentication in a computer system."
Backdoor definition: "A backdoor is a method, often secret, of bypassing normal authentication in a computer system."
That someone might not be the company, it might be a developer.
It's entirely true that the company says it's not a backdoor, the developer says it's a mistake, but he/she was approached from an external organization.
Unless you can provide either way it's impossible to classify it as a backdoor or not.
And considering you can never know if someone else knows about it, that means you can never know if it was a backdoor.