It is clear from the way that the boy reacts to the cornucopia of supplies that they find in this bunker that he certainly fears that what he is seeing is too good to be true. The fact that they just happen to stumble across the bunker as they are nearing their deaths through starvation is certainly something that appears too good to be true, and the way that the boy merely sits on the bunk bed shows his utter disbelief:
He looked at the boy. The boy was sitting quietlyon the bunk, still wrapped in the blanket, watching. The man thought he had probably not fully committed himself to any of this. You could wake in the dark wet woods at any time.
The boy, perhaps like the reader, is not fully prepared to believe in the reality of the bunker and the vital lifeline that it offers. However, the father explains to the son when he asks him if it is real that the bunker "is here because someone thought it might be needed." Given the panic buying that occurs in supermarkets nowadays with the slightest rumour of a disaster or problems, it is not beyond the realms of belief that somebody living in this future scenario would have predicted the disaster and therefore would have created a bunker for himself or herself and their loved ones. The existence of the bunker is not what is unbelievable, but the serendipitous discovery of it precisely at this point in the narrative can be seen as being slightly unrealistic. However, perhaps McCarthy is making a point about the forces of good and the hope that can exist even in the darkest of situations. Good things do happen, even in the midst of evil.