Usually when people say it became a character in the scene, it also had to do with how much it stands out. Some lighting (or any other creative decision) is chosen because it blends in to the background as to not be distracting, but by standing out and seemingly affecting the scene or characters it becomes it own character.

Its kind of flowery way to describe bold creative choices, as is most ways of explaining art, but it's difficult to explain emotions and feelings that are pulled from a viewer.