We really don't know, of course, how giant squid see. But we do know that they have huge eyes. Their eyes are about as big as a human head. It is typical for deep-sea animals to have very large eyes. And the assumption is that these eyes are so large so that they can gather the small amounts of light that are available.
That being the case, with such huge eyes giant squid must be able to see not only the remnants of light coming down from the surface, but also they must be able to see bioluminescent light -- that is, the living light that's created by other living organisms in the deep sea.
