I really hope it's not true, because that would just mean he doesn't understand how elements are placed on boards - there will always be some air unless you add extra filler. (and make everything run hotter)
> there will always be some air unless you add extra filler. (and make everything run hotter)
Adding filler is how you make a passively-cooled device run cooler. Air that's not moving doesn't transport enough heat to be useful, and convection doesn't do anything in the narrow spaces between and above components on a circuit board. Thermal paste, graphite sheets, and vapor chambers are what phones these days do with volume that's not needed for circuitry or structural support.
It depends on the filler and whether you want other things around to get warmer or not. Radios for example are normally kept away from too much heat, then any temperature difference is monitored/compensated in configuration. Anyway, what I mean is - that heat is going somewhere - any extra filler will change where / how quickly.
In the context of how phones today are built, air gaps are pretty much never the right answer to a thermal concern. At best, an air gap is what you allow when the budget cannot accommodate a better solution. If you have a component you need to keep cool, it's probably because it puts out a fair bit of heat on its own, so you want to thermally couple it to its surroundings. If you have a component you don't mind getting warm, you use it as part of the path for heat to be conducted from the big heat generators out to the surface of the phone.
