the problem with the holocaust deniers (and all conspiracy theorists actually) is that they pick parts of history or facts that were at one time believed (but that modern studies have shown to be wrong) as evidence of the 'holes' in the true fact that they are stating the conspiracy theory about.
for example, in the years following the holocaust, it was believed that 4 million people died at Auschwitz. in fact, it was on a plaque at the holocaust museum there for years etc.
however, modern scholars do not believe this figure and can verify that AT LEAST 1.5 million people died there.
the problem is that the nazis tried to hide evidence of their crimes and DID NOT keep records.
holocaust deniers love to point out the 4 million figure as wrong, and use it as evidence in their theory. they NEVER point out that the figure is now regarded as inaccurate and has been revised..