> I think the difference here is that an EV battery fire quickly spreads to the entire length of the bus

It's an interesting theory, but I don't see it supported by these articles.

The entire diesel bus had its back third engulfed very quickly, from these sources.

The entire electric bus burned, but we don't have any information on the rate at which the fire propagated.

If anything an EV should be possible to design to be safer against fires, as it's the battery itself that burns. A metal fire break on the floor and a gap between that and the battery should give warning signs (smoke coming out the sides) and give people time to evacuate before it becomes too serious. Temperature sensors throughout the battery could give warning signs even before it gets to that.

In an ICE vehicle it's the vapour of the fuel that burns. The engine of buses/coaches is typically under the floor, with access panels, so vapour can easily get into the cabin.

No, the bus batteries involved here do not propagate quickly. When it can't be contained, it takes more than 20 minutes to spread, which is plenty of time to evacuate the bus.