The Illusion of safety was a concept I came up with after years of studying aviation safety as a mere flight instructor.
In general terms, it means basically what that author says, which is briefly summarized by a Mark Twain quote - "It ain't what we don't know that gets us into trouble, its what we know for sure that just ain't so."
Perhaps a couple of simple examples will illustrate this point better. Since accidents still involve stalls and spins, there's good reason to suspect that pilots aren't trained to instinctively know when the wing is approaching stall, or how to keep the aircraft in balance without reference to the balance ball. Therefore, by teaching pilots how to recover from stalls or spins as we've always done, they're actually lulled into a false sense of security, thinking they know enough to recover, so no extra consideration or training is required.
Another simple example involves passenger screening at airports. The average pax may be impressed when their nail file or knitting needles are confiscated, thinking it's a sign the airlines are serious about their safety, or when flight attendants wake them up around TOPD to ensure their seat belts are fastened, tray tables stowed, and seatbacks upright, but there's really no safety benefit from doing any of that just after leaving cruise, or even before landing as long as ops are normal.
Too much is sold under the safety banner that doesn't seem much individually, but collectively distract valuable attention away from things that really matter.