Emissive surfaces are expressed as cd m2 (nits). A sky box in hdr is using the pixel intensity multiplied by the light intensity, resulting in a total luminance, expressed in cd m2.
Think of the hdr pixels as a filter . If your hdr image ranged from 0 to 1.0, and the sky set to 1000 cdm2, the resulting luminance would be 1.0 x 1000 cdm2.