I wonder if he was concerned about 'breathing' for temperatures. Generally there are parts of the car which can heat up under heavy use, and you may be concerned about the heat dissipation qualities of paint. Sure, BBQ paints (as well as others) work well for withstanding heat, but not necessarily for allowing heat to flow.

So he may have felt that under heavy abuse, the tranny/transfer case heated up, and was attempting to 'breathe' (cool) across its surface area, to which paint may only add a layer of insulation.