Literally in what world does Light and Darkness not count as a duality when Darkness is literally not light in the first place?
Because darkness is just the absence of Light, of course, you could use different way to wording the absence of Light as Not Light, but that is just playing with words, just using different wording but still have the same meaning. Currently nonduality standard follow truth value and valued logic, which is truth and false, is and is not, for example A is truth then not/non-A is false.
Kinda nonduality standard currently follow principle of logic. If something, such X logically exist, its negation non/not X, should also exist to negate X, if you remove either X, its negation non X will cease to exist too, it is called logical negation. Hmm, think about number 1 for example, you could write its opposite -1, but if number 1 do not exist, then -1 also not exist either, because there is no number 1, you could only write the minus symbol without the number 1 after it, hard to explain i guess
Light and Darkness, Fire and Water, while opposite, exist in their own (logical) state, if you remove all of light or fire, darkness and water still there. If you use fire on water, water's liquid form will turn into gas form, gas form of water is still water, it is still existing, but in different state, which based on logic, allow it to exist. That why
@DontTalkDT and
@Ultima_Reality said, like a rock isn't life or death, but didn't make it nondual either, cause the rock exist in its own state of existence, and in a logical way, logic allow it to exist