I understand you want to counter this statement with physics, and we could have a rich debate about what we know of the universe and how light and it’s absence exists in it. But I think you misunderstood what we are talking about.
This statement is about philosophy, light and dark are metaphorical here. We could just as well say “up cannot exists without down”, or “day cannot exists without night”. The next step to this philosophical thinking is to realise that since one cannot exists without the other, therefore they are the same thing.