What does the narrator mean when he says "it's the truth even if it didn't happen?"

Chief Bromden is an interesting narrator. Although his mind is often in a "fog", he comes across as more lucid than the average person. Chief often hallucinates things but his images serve to add greater clarity to the truth. That is why he says, "It’s hard for me to have a clear mind thinking on it. But it’s the truth even if it didn’t happen."