In the time you drive to the station, fill up and drive home, you’re still on the old gas in the tank. Your tank is not bone dry when you get gas. There’s a few litres in the bottom and that’s what you’re running on because fuel pumps draw from the bottom.
You could travel quite a ways before you start draw in that diesel, so if you stop before any of it gets into the fuel pump, it’s much better. All you need to do is drain the tank, rather than dismantling and flushing the entire fuel system. This is the point officials are making.
All that said, the actual danger to the engine here is a lot more nuanced. It depends how much diesel went in. Diesel is actually a light oil, unlike gasoline. A little diesel in there would be equivalent to a leaky head gasket. The car is going to smoke a lot and run poorly, but it’ll run and you aren’t gonna brick it immediately doing that. At the other extreme, if the engine suddenly got 100% diesel, it’s going to stall anyway. Diesel doesn’t ignite from a spark the way gasoline does. It requires extreme compression which is how diesel engines work. The power stroke will fail on each cylinder in turn, and that’ll be it. It’ll stall before a lot of damage is done. Somewhere in between there is probably a mix of gas and diesel that would do a lot of damage if you let it run.
Their blanket advice of “don’t start the engine” is easy to say and easy to follow and guarantees no damage, so that’s why they say that. Public safety announcements are no time for nuance.
