I agree, The thing that hangs me up is it would just have to be mirrors. The amount of time it would take to do that amount of work would have left at least a 1/4” of accumulated soot if any available flame methods of illumination at that time were used, low soot or not just because of the amount of time it would have taken it would have accumulated at least some.
Had to be mirrors or some other unknown “method”. :)
Your estimate of the soot seems far too high; probably your estimate of the time needed to carve out those KV tombs for example; regardless, if there were a visible accumulation, moisture and old father time would have delivered almost all of it onto the floor, along with the erosional deposits and flaked off paint, bits of mummy, that kind of thing. And that's assuming the last task of the workmen wasn't cleaning off any accumulated soot as they left the tomb.