I don't think anything has been filled in.
The older image is taken from the end of the basement sheet, on which the descending passage was built on, so we are looking directly at the entrance.
This area had been stripped of its masonry in antiquity, and if you were allowed to climb up there today, you would see much the same view.
The modern image is taken from the pyramid base looking up, so the entrance is not visible from this angle, i.e. the opening in which the person in the old image is standing, simply cannot be seen from the ground.
Hope that helps