For context, that note is specifically calling out roof decks, not all metal generally. In the construction they're describing I believe the foam would be going on the outdoor side of the roof deck. A common approach to commercial flat roof design is to build a deck with wood or metal, then layer insulation board on top, and cap it all with a membrane to keep the water out. It's with that lay-up in mind that I would guess the note is concerned with the potential for a fire below the roof, ie inside the building, to heat the roof deck enough to melt or even ignite the insulation on the other side. A thermal barrier would prevent or at least delay that outcome.
IMHO if there's a fire raging up against the metal on the outside of my bus skin, melting or ignition of the insulation pressed against the inside of the bus is probably low on my list of worries.