Well I wouldn't say they aren't immortal since some died throughout the story, I think the best way to say is that they just don't age. They're like vampires they don't age but still don't die no matter what unless with specific attacks. If they were truly immortal they would never die no matter what whichhh seems to be whem the light is taken from their ghost (in the early missions on the moon) or in regular ways? Like the Rise of Iron squad.