You're right. But a good engineer also allows some margin in any design for the fact that bad/dumb things happen. I wouldn't consider it unreasonable to expect failure if we were talking about a 50% or greater concentration of gasoline. But less than 20%? Gimme a break. ANY engine should survive that briefly (not continuously operated that way), and any gasoline engine should survive that much diesel in its fuel.
And it isn't just moronic owners. Delivery tank truck drivers have been known to put the wrong fuel in storage tanks, you know.