Heat doesn't cause gas to ignite. Spark does. He's already dumpin gas into a hot intake straight outta the carb. I have done a BUNCH of them either in the air cleaner or the valve cover breather, whichever is easier and never had a problem. I know you have been doing this longer than I. Have you ever really seen that happen before?
Open your hood at night. Some have a circus of sparks from the wires. The alternator is a constant source of spark. The heat just caused the fuel to boil and the fumes are very explosive when the hood is closed.