Backdoors need not be literal. A well-misplaced if will go a long way in leaking a key information. Hardly a "backdoor" in a more common sense of the word, but an exploitable weakness nonetheless.
An intentionally introduced vulnerability can be considered a backdoor, even if it's not a matter of saying "open sesame" to open the so-called backdoor.
While I doubt it's an intentional backdoor, I wouldn't assume that backdoors would be obfuscated. You can't deny knowledge of an obfuscated backdoor, while an obvious one could plausibly be a simple mistake.
A vulnerability is not as damning if it wasn't intentionally inserted to be a backdoor. It being easily discoverable suggests that there was no intention to hide it, and thus it was not intended to be a backdoor.
If conversely the vulnerability was difficult to find externally, was publically unknown, and was seen being used for attacks, that would be suggestive of a deliberate backdoor, and far more damning.
The problem is that a backdoor might not be so obvious. It can be simply a wrong chosen algorithm key size, and you need to be a cryptographic expert to know that.
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