I have had a couple trucks towed in with full tanks of gas which only ran for a minute or two. They too got flushed out and refilled without issue to this day. I am not sure how quickly damage is done, some say immediately, but it seems the factor here may be how long the engine was run with gas. If the tank is filled when there is still a decent amount of diesel that would allow the engine to run I think that scenario is the most detrimental.