Using polished metal mirrors has been shown to work well enough at surprising depths. Also, the oils (olive oil, seed oils) used in ancient oil lamps were also very low producers of soot. Kerosene is a modern discovery, if memory serves was discovered in Poland (the discoverer had what turned out to be oil seeping out on his property, and his neighbors urged him to try to make vodka out of it; that could easily be a mythical detail).
http://oliveoilsource.com/asktheexpert/how-did-ancients-use-olive-oil-their-lamps
https://www.antiquelampsupply.com/type-of-oil-used-in-oil-lamps
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”. :)