[jerf]

You've got it backwards. They are not saying all life-bearing planets must have chiral molecules, and therefore we can tell whether a planet has life based on the light. They are saying that if a planet exhibits light coming from a source with lots of chiral molecules, then the only known explanation is life, and we can conclude it is life. A non-chiral source proves nothing either way.

Note this same logic holds for oxygen. This expands our ability to positively identify life to places that may not have gone the oxygen route. (We know this is possible, because Earth itself was like that for a while, and we continue to have a wide variety of anaerobic organisms, and even some complete anaerobic biomes underground, I believe.) It does nothing to our ability to negatively identify life (or positively identify lack of life, if you like), which we have little ability to do in general.