Just thought I'd go look it up and see if I could calculate how many btu it would have taken to cremate 6 million people. The numbers are astronomically high.
If we used compressed natural gas to cremate 6 million people it would take 1,500 full ships of compressed natural gas to get the job done. Assuming it took 32 m^3 of CNG to cremate a human, 6MM humans to be cremated, each ship holds 125,000 m^3 of natural gas. And they didn't even have modern CNG ships back then.
I don't know where they'd get that much fuel in a war that devastated Europe and the German Army was running on wood gas for the last half of the war.
Just a simple mental exercise to blow the Holocaust narrative right out of the water.