I think it is possible that the pyramid is not made of millions of blocks, but the outside is several layers of blocks with filler. It would explain the logistics.
Thus this might not be a room, but filler of lower density.
There is no filler in the Great Pyramid. Were there some, the "robber's hole" that was dug up from the descending passage would have encounted it. All blocks, all the way up to the Grand Gallery.