This is not an unknown phenomena. I've encountered online and offline, several that deny the whole holocaust ever took place. Some have even denied the wars ever took part. Some, as in Vietnam maybe still fighting a war long over.
Take Teruo Nakamura, he refused to believe the war was over and he continued the war until he finally surrendered in 1974.
If you look at Japan's propaganda campaign used by them in wars you can begin to see why some people think as they do. It is also well known history is re-written. Propaganda is often used as a tool. News reports only what it wants or is told to report. Our own government hide or alter evidence, can we honestly hand on heart believe what is said to be true as the truth?
For example, if I take the Propaganda campaign and state, there are no Doctors. I also re-write history to show there is no Doctors. Then ask are there Doctors?