It's the petrol vapour that will ignite and being under a closed bonnet you may have had a nasty surprise if that vapour had ignited with the manifolds being so hot.........the KV6 seems to generate a lot of under bonnet heat after a good thrashing anyway
Methinks you had a very lucky escape.........otherwise you may have had a flame grilled ZS instead of just a whopper meal.
Smitten Jim