Not really... backdoor just means something that can bypass the main security.

It's just that publicly known backdoors are of very limited usefulness, because people go out of their way to remove, disable, or avoid them. Or worse, use them for their own gains. There have been more than a few cases of governments trying to implement and enforce publicly known backdoors (with keys only the government knows), such as the Clipper cryptography chip in the 90s.

But... just because something is documented, doesn't mean it's publicly known. We have an example here of something obscure enough to be a useful backdoor (assuming someone knew about it).

And while the underlying feature might have been documented, the fact that many BIOSes enabled the feature was not documented anywhere. That does count as hidden.