Stalin was guilty of having death camps and gulags of his own. The USSR was up to a whole lot of shifty stuff during that period that they did not want the rest of the world to know. It is not absurd to suggest that Stalin attempted to distract the world with "Hitler's atrocities" so that his own evils would not be realized. It is not an unreasonable theory seeing how conveniently only Soviet-controlled camps "were death camps".

Basically it boils down to -- you can't trust the Soviets, and all of the "evidence" for the Holocaust rests on the word of Soviets. Not a compelling combination.