Beware that reasoning. We only knew about the holocaust very late into the war and there were denials just as you claim right now.
And indeed, when the Nazi's were losing they tried quite hard to hide the evidence, however it was so total and immutable in many cases that it could not be hidden, mostly because the allied powers controlled their lands and had free access to their previously governed population..
Without such access proof rarely escapes and when it does the total brutality is considered impossible; this is exactly what happened during world war 2.
