Most people have a very poor understanding of what 'holocaust denial' actually is. People denying the holocaust are not denying that Jews were rounded up and put into concentration camps, where they were forced into labor and unsavory conditions. It was very similar to how the USA rounded up the Japanese into concentration camps, and for very similar reasons.

Holocaust denial is instead a denial of the founding mythos that demonically evil Germans gassed 6,000,000 Jews. There is significant evidence against both the number supposedly killed, and the existence of death camps which used gas and other means to mass exterminate prisoners. The number itself, 6,000,000 was actually not settled on until years after WWII. Couple that with the fact that it is well documented that the Nuremberg "Trial" confessions were coerced through torture, and there is significant reason to doubt the official holocaust narrative.

Holocaust "Denial" is generally a belief that delousing showers created the rumor of "gas chambers" among camp members. The typhus outbreak was what forced this delousing process, and also what caused the pictures of the very gaunt looking Jews in the camps, who were primarily Typhus victims. Post war, almost every camp the USA liberated was found to be a labor camp, and meanwhile most "death camps" were liberated by the soviet union. In an effort to cement into the public mind how evil Germans were, the rumor of death camps was exaggerated to both justify the war effort, and to hide allied atrocities. Realistically around 300,000 likely died, primarily due to Typhus, and the fact that allied bombings made it very hard to get food and resources to prisoners.

If the USA had been invaded, the Germans could have easily said that we "holocausted" the Japanese.