I understood that to mean the oil was so special and awesome that it glowed with light even before it was lit.
Well, precisely. That is what it says. But if you think about it, the oil is the only thing in the verse capable of actually producing
We have a lamp in a niche, neither of which are capable of producing light on their own. Instead, they are places from which the light can emanate.
Then we have the glass which is compared to a brilliant star. But the glass, of course, cannot produce its own light. It refracts, reflects, and intensifies the light – so much so that the glass itself ends up looking like a glowing orb, a brilliant star—but the glass itself is not the source of the light.
Then we move on to the tree that fuels the fire. This is where it gets really interesting because, as mufassiroon have noted, the olive is both a blessed tree and a humble tree. Short and dull, yet extremely versatile. Used in both religious ceremonies and daily, mundane tasks like cooking and grooming. Holy only because we deem it to be. “Neither of the East nor of the West is almost a way of saying, “neither here nor there.” It exists just as a worldly tree would, but not in any way that we could understand.
It is certainly not the wood of this tree that fuels the light. Instead, it is the rich, plentiful oil – its essence. The essence of God. So rich is this oil that it nearly shines on its own, even if it is not lit. If the oil can nearly shine even without being lit, then it’s almost as though we are being told that it does not matter if the light really exists or not, you don’t actually need it to produce the light. Through the niche, the lamp, and the brilliant glass, the essence of this oil, which is inclined to shine on its own (though we know that oil does not actually
shine) will produce light. Something from nothing.