I'm just speculating that when you believe in and promote something with no empirical basis, it would be natural to fear having your beliefs shown as groundless. Not an obvious fear on the surface but a deep seeded one that you would only become aware of by paying attention to your sensations, preconceptions and feelings.
I've noticed this in myself various times in different contexts. I may have a feeling that I'm digging my heals in about some issue, but when I examine my faculties I realize that I'm reacting this way because I don't have all of the facts and my ego is perhaps digging in. I'm describing this ego reaction as fear.
But for those who are promoting a "noble lie" in order to achieve certain ends, having their falsity exposed could truly be dangerous and worthy of fearing. At the very least it would be a threat to their power, which such a person would presumably fear.
Also, when I wrote that comment I was specifically thinking of religion. The fears inherent in that are, of course, fearing gods and death, as well as what I've just described.