The context is Ven. Dhammapala saying nibbana is full of light, because there is no gloom there. Gloom, sun and moon are conditioned. There being no gloom, sun or moon there is instead unconditioned light, which is the intrinsic nature of nibbana (Ven. Dhammapala’s words, not mine). No one is saying that the light described here is physical. That looks like a straw man.
Saying that nibbana has the nature of light is also consistent with the wider context of CT, since nibbana isn’t nothing according to CT. It’s really some-thing. There is existence in final Nibbana, according to CT.
(Edited by mod to remove unnecessary comment)