A different way to think about it is the Newtonian approach: your wing is accelerating air particles in a downward direction, and the equal-and-opposite reaction is what (partially) generates the lift on the aircraft. Lower density means there’s fewer particles to deflect downwards in a given volume of air. In a similar vein, a rotation of the propellor accelerates air particles rearward, which results in a forward thrust on the aircraft; fewer particles to accelerate means less thrust.