In connection with the above, let me quote a different point made by Mattogno.
It is not likely that, nineteen years after the alleged event, Widmann could have remembered the details he described in his 1960 testimony. The most obvious explanation is that he, too, was shown the photographic sequences of the film and did nothing more than put these images into his own words and pass them off as his memories, but this did not prevent him from making some mistakes.
The Einsatzgruppen in the Occupied Eastern Territories (2024) by Carlo Mattogno, p.317
I had exactly the same impression. When I read Widmann's testimony it seems as if he describes what is shown in the video and little to nothing else. Here are some example quotes, borrowing liberally from sources mentioned upthread.
On the afternoon of this day, Nebe had the window bricked up and two openings for the gas pipe prepared.
[separate interrogation]
The window of this room was walled up, two holes were spared to introduce the sockets for the gas-tubes.
The video does show a window that was bricked up with two gas pipes in it.
There were initially no patients in the laboratory. They were carted in shortly afterwards using a horse-drawn cart. I can only remember one such horse-drawn cart; there may have been room for about 5 or at most 6 patients on this horse-drawn cart, who were brought into the laboratory.
[separate interrogation]
Then the mentally ill were led into the room. I cannot tell how many. They were brought with a panje wagon, I estimate it had room for 5 to 8 persons.
Why yes, the video does have a scene showing exactly 5 people on a cart with room for 1-3 more. In another scene, 4 are visible on the cart. We cannot tell how many total there were by watching the video because they are not pictured.
With the help of a Russian male doctor and two female doctors a room was selected in the asylum [...]
The video shows a male and a female, described by the USHMM as "purportedly Soviet doctors". Could someone watching this extrapolate the existence of a third doctor?
Nebe subsequently had another hose connected to a personnel van belonging to the ordinary police.
[separate interrogation]
Thereupon a truck engine was connected to the second introduction socket and started up.
Again, two pipes connected to two vehicles is exactly what we see in the film.
As I already made clear, the building was covered with white plaster and had a foundation block.
The building was predominantly exposed brick, but we do see a section in the video where plastered brick appears. Did Widmann remember that detail and mistakenly apply it to the whole building?
Moreover, one of the two hoses we had brought with us was much thicker than the other one.
In the video, the section of pipe coming out of the back of the truck is noticably thinner than the pipe it runs in to. The hoses don't appear to be identical, either.
[...] a room was selected in the asylum, it was a small lab close to the entrance door. [...] Nebe observed the process in the room through a little window and noticed after 5 to 8 minutes that there was no effect.
[separate interrogation]
Nebe went into the building where one could see into the laboratory through a glass window in the door.
He offers only these two details about the arrangement of the inside of the building, which was not visible on film. An interior window and the room's proximity to the entrance. Possibly he was told about the "little window" from the interrogation of Andreas von Amburger who described it as "specially placed slits". Hans Metzner quotes him saying that in 1945.
So in net, Widmann says almost nothing about this event that isn't already visible in the 1947 propaganda film, plus one detail that could have been copied from a prior witness.
That was all in January 1960. It wasn't until 1962 that Widmann, in response to being shown stills from the film, denied their content by volunteering several new details. I realize it's a strange contradiction to say that he first recited the details he saw in the video, then denied the veracity of stills taken from that video, but that is how his testimony appears to me. He seems to have been a cooperative witness in 1960. Perhaps something changed by 1962.