As I see it, the ability of the fuel to be ignited is a constant. IMO gasoline is the one to focus on since it is easier to ignite, and it's commonly present very near the diesel pumps anyway.
The variable in the experiment is whether the engine continues to idle, or if it is shut down and then restarted. Is one of those variables more likely to ignite gasoline fumes?
Is it possible the rules against idling are more effective in preventing inattentive drivers from driving away with the nozzle in the tank than in preventing ignition per se?
Most driver safety programs will attempt to ban idling with the drivers out of the vehicle as a general safety measure. A lot of things can happen with a running vehicle unattended, most of them bad. I worked for a company that banned leaving keys in electric golf carts because of runaways. Not because we were gassing them up!
