I used to be in the intelligence field, and I held a top secret SCI clearance for about five years. What most people don't realize about classified information is that it's not the information itself that's so sensitive; it's the means via which such information is acquired that must be protected. If this were not so, targets could simply sidestep our intelligence collection vectors. I rarely dealt with any classifie…
The article spoke distinctly of security clearances that you don't even know exist. You probably didn't have those.
Thus, there are likely many clearances/compartments that a great many people don't know exist or don't exist. I suspect there isn't really anyone who knows what every compartment is.