I can see their point. One can view obscurity as "not common knowledge" or "information that can be found if you look hard enough."

A secret key technically falls in the later. You have it somewhere. If someone looks hard enough, they can find it. It may take ridiculous effort to the point of being impractical, but that is still obscurity.

I recognize that when people speak of "security through obscurity" they are referring to esoteric knowledge that is difficult but not impractical to find.