[grahn]

The irradiance is determined by the aperture, which is measured relative to the focal length. So with e.g. f/2 you get the same heating of a given area of the sensor regardless of the focal length (i.e. "zoom").

Put differently, longer lenses typically also have bigger absolute apertures, collecting incident light from a larger cross-section. This compensates for the fact that a given solid angle of incident light is spread over a larger area of the sensor.