Plenty of people were sent to the gas chambers upon arrival at the camps, and plenty more died of disease and malnutrition while at the camps. Holocaust deniers don't want to accept the fact that the Nazis didn't have to murder a large number of prisoners outright, because thousands, perhaps millions of slave laborers died as a result of being overworked, and starved to death by the Nazis working on their various pet projects.
Several camps were outright extermination camps such as Belzec. Almost nobody survived arrival longer than an hour.
A few were kept out as workers and they were processed through as well at regular intervals.
No labor there, just straight out murder.
Then there are the million murdered naked shot beside a ditch from Poland all the way east into Russia an Ukraine.
The nazis were a sick lot.