The holocaust can’t be denied for the simple fact the Nazi never denied it. They said it wasn’t me but he (Hitler, Himmler and others who commit suicide if possible), the courageous supermen, but they never try to play this card.
People who try to deny it have the burden of the proof. They have to give the bills for concrete, wires and wood for the barracks. They have to give evidences, named the general in charge, provide the names of the companies hired to build the camps by the allies. They of course can’t, so they spread rumours, allegations etc…
Just read Speer, the only Nazi with remorse and main architect of Hitler, and that is enough.