I don’t know about the theory that the sculpture wasn’t originally contemplated and it was just a open air quarry for a temple that later took advantage of the rock in the “u” of the quarry.
Who ever saw a quarry built in a “U” shape? That isn’t a very efficient workspace IMO>
The usual take on the Sphinx is that it began as a yardang, or rather, that the head (and whatever has been removed above ground level) was a yardang.
The conclusion that it wasn't originally contemplated as a sculpture is imho, but based on the evidence. The area in front of the Sphinx was quarried out as well, but there's no telling how high that was. The techniques used to move 200 ton stones out of the Sphinx enclosure may have required the final appearance, also, the body of the Sphinx (much of which is covered with small 'repairs') may have been too flawed to cut into such large blocks.