He is just explaining the report away. He claims - without evidence - that this cremation rate of 15 minutes per corpse "had to do with a different project, though illusory, by Kurt Prüfer". So what project is this supposed to be? And what evidence is there that it has to do with a different project at all?
What it of course also noteworthy is that the report suggests that two male corpses are incinerated at once in one muffle. If we convert those "two males" into what were actually extermination victims we end up with three corpses of women and children cremated at once - as reported by eyewitnesses.
Mattogno's response to Prufer's statements are here:
http://www.vho.org/GB/Books/anf/Mattogno.html
See Chapter 3, Section 2 for capacity, but the entire work is worthy of a read.
This is about Prüfer's interrogation, but not about the memo he wrote in September 1942 according to which the cremation capacity of the crematorium in the main camp was 250 corpses per day, or 35 min per corpse when related to 24 hours or 30 min per corpse when related to 20 hours. You will notice that this figure by the oven builder - long after they were already in operation - is roughly half the time Mattogno claims.
At the end of all of this, the missing element remains; Where are the ashes of the alleged millions of people cremated? Average male and female is roughly 4 and 6 pounds of ash, respectively. Roughly a shoebox full. So I'd like to skip ahead and know where the millions of shoebox's worth of ashes went, especially considering the food and water supplies weren't contaminated from the alleged human remains.
I don't see a problem. Ash is a fertilizer and only a fraction is actually soluble in water. Most of the ash should be metal oxides that are present in the ground in a similar form anyway.