Wouldn't an F-35 need a paved, smooth runway? The A-10 is expressly designed (for example, the high-mounted engines to help avoid FOD) to operate from rough ground.

The engines on the A-10 were placed above and between the elevator/vertical stabs and the wings in order to reduce IR signature for missile avoidance. "Unimproved airfield" was not a specific criteria, and not the "doctrine" for the airframe, but later realization that such operation of several aircraft on "improvise runways/airfield operations" became tested and fielded as necessary.