That post is wrong though. The real answer is, as usual, "it's complicated". But not very so. The beam width is roughly constant with distance close to the fo

That post is wrong though. The real answer is, as usual, "it's complicated". But not very so. The beam width is roughly constant with distance close to the focal point, so there the inverse square law does not apply. Far from the focal point, it's the 3D angle that's roughly constant, which means that the law does apply there.