Having changed the clamp I don’t think it was the cause as when I re-applied the shader there was still low level illumination (see below). Also, I’ve linked here a postprocess with JUST diffuse plugged in and you can see that there are no lights in the scene and yet there is still diffuse light color data being displayed. My next intuition is that the postprocess material must be providing SOME illumination but if I remove the shader the scene displays as if there were no light, so even an incredibly simply shader that just uses diffuse light color data as its only node still artificially lights.
With Clamp 0:
With Just Diffuse:
Also, my above point about origination of light seems a bit worrying I don’t see how previous people in this thread could have been calculating the direction of the diffuse light using only that and the world normal (which is static as far as I can tell).