Certainly if the writer intended it to be an accurate narrative of the creation of the universe, it would be enough to show that the writer didn't understand that light came from the sun. On the other hand, if the writer instead intended it to be a sytlised or metaphoric narrative, one cannot come to that conclusion.
I can easily imagine why a writer, putting together some sort of metaphorical account, would create light on the first day in the way that it is done. The account describes the process in terms of what is done on each day. Obviously, in order for there to be days, as understood at the time, there would have to be a light/dark cycle.
I'm having more difficulty understanding a metaphorical or stylised reason why the sun, moon and stars were not created until the fourth day, but literary exegesis was never my strong suit. In any event, as I said previously, the mere fact that daylight came three days before the light sources is not enough to convince me that the writer didn't understand that the sun was the light source.
I agree that a naive viewer might think that the day sky was itself a light source, but I don't think it necessarily follows that that is what the writer believed.
Those who would sacrifice an essential liberty for a temporary security will lose both, and deserve neither. -- Benjamin Franklin