"It's documented in the datasheet" is such a weak excuse for a backdoor.

Documenting a backdoor doesn't make it not a backdoor, just means it's not a hidden backdoor.

The fact that a number of machines shipped with the backdoor accidentally enabled, and nobody noticed for over a decade shows just how dangerous even a documented backdoor can be. The oversight wasn't even detected by someone reading the manual, it was detected by a security researcher who wrote a generic tool to fuzz out such backdoors.