No what I mean is, lets say you are setting up an actual "physical" security system for say, a bank.
One of the features you use are motion sensors along the floor at night, so anyone walking across the floor triggers the alarm. But your competitor sets up a security system in the bank across the street, and places motion lasers across all the doors and windows.
Then he promotes his security system against yours by telling people to place their arm through the window of the bank your system guards. When the arm through the window doesn't trigger the alarm, he claims triumph, says your security is inferior, because his alarm would be going off and yours is not, even though if any of those people had set foot on the floor that you had motion sensors on, the alarm would go off, the bank was protected the same.
The metaphor is a bit long-winded, but the principals are roughly the same. What I meant by a proprietary facet is a feature of their program that is nearly exclusive to them (motion sensors on window). So they make a special tool to test this feature (arm-through-window test), knowing nearly everyone else is going to fail. But that doesn't mean they have less security, only different security. Now, Avast! for example might indeed fail to notice this tool looking at your documents. But where Avast! kicks in is as soon as actual infections try to phone home. As this tool likely just looks but doesn't try to phone home, Avast! leaves it be.