I'm fairly certain that by "incredulity", McLuhan meant "unwillingness to believe", but if you look up the definition of the word, the other primary meaning is "inability to believe".

The lack of that ability is, I believe, at the heart of the matter. Could someone lack such an ability, like some sort of intellectual cripple? What would that mean, precisely? Is there an actual mechanism at work? A defect in your leg makes you a physical cripple, so is there a defect in consciousness?

McLuhan hit the nail on the head but his hammer had no force behind it. If he had had these questions, he certainly would have amplified and followed up on them.