One of the interesting things about the holocaust is that if the eyewitnesses go into any kind of detail at all, no two of them describe the same thing. This writer claims to have worked at Auschwitz under the infamous Dr. Mengele.

AUSCHWITZ: A Doctor's Eyewitness Account, by Dr. Miklos Nyiszli

Dr. Mengele, the medical "selector" made a sign. They lined up again in two groups. The left-hand column included the aged, the crippled, the feeble and women with children under fourteen. The right-hand column consisted entirely of able-bodied men and women: those able to work. In this latter group I noticed my wife and fourteen year old daughter.

Now here is a doctor working at the camp and does not appear to have the connections to keep his family out of the camp.
We no longer had any way of speaking to each other; all we could do was make signs. Those too sick to walk, the aged and insane, were loaded into Red Cross vans.
Here the writer is the only witness to ever mention the use of Red Cross vans. Witnesses from Auschwitz usually say the people walked. This is the lone report of being carried by van.
(This Dr. Nyiszli worked for Dr. Mengele doing disections and other medical work. This is after a gas chamber is filled with 3,000 naked men, women and children.) ...
This is the only report of men, women and children together. The usual report is that men were separately gassed from the women and children.

Note also there is a claim of 3000 people. Note that the size of the rooms presently identified at Auschwitz could not possibly hold 3000 people.

The doors swung shut
This suggests clearly that there was a double door entrance to the building from the outside but none of the buildings presently identified had double doors. Also those buildings had five interior rooms.

Thus again we have a description detailed enough to know that it matches no identified building. We are asked to believe in gassing in buildings that no witness describes correctly.

and from without the lights were switched off.
It is unclear what point there was in turning off the lights other than to leave the people in complete darkness. Perhaps it has to do with satanic rituals in the dark as a demonic storytelling attribute.
At that very instant the sound of a car was heard:
It is unclear just what point the author is trying to make with this remark.
a deluxe model, furnished by the International Red Cross.
Here is another report of a Red Cross vehicle. It is also the only such report.
An SS officer and a SDG Deputy Health Officer stepped from the car. The Deputy Health Officer held four green sheet-iron canisters.
Zyklon B containers had replaceable lids, as in paint cans, and red and yellow labels. It is unclear what this might have been or what a "sheet-iron" might have been from this translation.
He advanced across the grass,
The only building in the entire Auschwitz-Birkenau complex where this could have been possible is on the morgues associated with two of the crematoria. They were largely underground and had flat concrete roofs. The drawings show dirt bermed up against the exposed walls but aerial photos do not. Dirt might have been put on top of the roof and grass planted. There is no indication of the latter having been done other than this report. It does not appear in the aerial photos.

If this was done it was prior to the aerial photos and then removed prior to the Summer of 1944.

where every thirty yards short concrete pipes jutted up from the ground.
Here we have four concrete pipes set thirty yards apart indicating a minimum length of this underground room of 90 feet, more likely indicating 120 feet. This appears to suggest the building was the morgue.

The problem with it being the morgue and the 3000 people is that the "gas chamber" is no longer identified as the entire building but rather one room within the building. Yet the only way to even think of getting 3000 people into that building is to use all of it.

Having donned his gas mask, he lifted the lid of the pipe, which was also made of concrete.
Unfortunately for the writer, there are no signs of any such concrete pipes in the remains of the morgues.
He opened one of the cans and poured the contents--a mauve granulated material--into the opening.
Whatever this granulated material is, it is not Zyklon B or any other form of cyanide. The color is wrong.
(The Dr. writes that within five minutes everyone was dead).
This is one of the faster times to death ever reported. But note that the man pouring it in had to remove a concrete cover, open a can and pour it in, he has exactly 75 seconds for each can just to reach the last pipe and pour in the last can just as that last person dies.

This clearly indicates they were using way more than they needed as Zyklon B requires hours to outgas but then, from the other descriptions, this is clearly not Zyklon B. What it is, is unknown.

...For every convoy it was the same story. Red Cross cars brough the gas from the outside. There was never a stock of it in the crematorium. The precaution was scandalous, but still more scandalous was the fact that the gas was brought in a car bearing the insignia of the International Red Cross.
It is unclear what the point of using an IRC vehicle was in that the people were already inside an underground building.

      Thus this writer, while claiming to have actually worked at Auschwitz and to have witnessed what he writes about has managed to get nothing right. It is this sort of material that writers about the Holocaust would rather sweep under the rug.