Light here doesn’t mean light like we get from the Sun. It has nothing to do with physical light.
It is referring to its utterly beneficial nature as the contacting of it by magga citta and phala citta eradicates defilements.
Nibbana is the antithesis of samsara. It has not a spec of conditioned phenomena (such as physical light).
The Udana Commentary p. 1013
By Dhammapala
Wherein there is neither earth, nor water" and so on so as to indicate its own nature via an elimination of things that are the antithesis thereof. Herein:
Just as nibbana is nowhere ( to be found) amidst conditioned (sankhata) things, since it has as its own nature that which is antithetical to all formations (sankhara), so are all conditioned things (not to be found) therein either for the collocation of things conditioned and unconditioned is ( a thing) not witnessed.
This is the explanation of the meaning in the present case: wherein, in which nibbana, in which unconditioned element, there is neither the earth-element whose characteristic is that of hardness, nor the water element whose characteristic is that of oozing, nor the fire-element whose characteristic is that of heat, nor the wind-element whose characteristic is that of distending. Hence,just as, through this mention of the absence therein of the four great elements, the absence of all derived materiality comes to be mentioned,