Rudolf reproduces data from the Zyklon manufacturers, showing that Zyklon does not spread very fast in stagnant air (no people).
Sorry, this is just outright false and bogus. Anyone with even a simple understanding of Zyklon-B behavior knows that the evaporation process creates a chemical reaction that produces an overpressure. This overpressure pushes the gas cloud out in all directions as it seeps and penetrates. That's why they use it for fumigation.
This overpressure forcing is best illustrated in the delousing chambers where the strength of the overpressure, combined with the seeping-conductive quality of cyanate reactivity with materials, caused Prussian Blue staining on the outside of the chamber wall. Circulatory fans aided this process, but were in addition to the overpressure distribution of the gas.
Still, the critical thing about gas chamber revisionism is the precise rate of overpressure expansion. Rudolf is the closest one to defining this. So, you are partially right that unaided expansion of cyanide gas wasn't fast enough to kill in the prescribed 5-15 minutes. Rudolf established those parameters - which is why they blocked discussion of them and slammed him in jail for the prehistoric crime of "holocaust denial". These same jailers never discussed his gassing parameters.