He describes the tomb as a cenotaph because the story on the walls goes full circle, ending with the resurrection of Harwa. There is no sarcophagus or coffin.
And after a few hundred years the tomb was used to hold lime-kilns used in destroying the bodies of the victims of the Plague of Cyprian in the 3rd century AD.
I won't try to paraphrase the lecture here. I suspect it will be on YouTube by now! And I apologise for the poor quality of some of the pictures: I did not zoom in enough, and the autofocus occasional didn't work. Instead I've added a YouTube video of an earlier talk on this subject.