Superchargers are likely as well, but they require additional space to be installed, which although isn't that much of a space, I'm not sure is possible to put on a drone which was specifically designed to be built around an engine of certain dimensions.
Secondly if the variant which holds the altitude record is same as one which holds endurance record, then a supercharger is counterproductive.
Lastly a little bit of tweaking the engine or fuel injection to boost its HP makes more sense financially for a private company, rather than waste money tacking on a supercharger, optimising and tuning the supercharged engine and the additional weight and change of CoG, then hoping to set a service ceiling height, that is far above the operational altitude (aka higher than anyone using the drone would fly).
Because I doubt anyone would care if it's 24,000ft or 27,000ft, because in the end you'll fly it at 18,000ft.