Fyron, once again we have the opportunity to disagree.
Life is sustained by a series of chemical reactions which take place in a solvent, which for all known life is water.
I agree that chlorine is incompatible with water-based lifeforms, so a chlorine-breathing creature would need to use another solvent, for example trichloroethylene.
AFAIK, chlorine-breathing lifeforms are not impossible, and in fact are more plausible than CO2 or argon breathers.
There are a few SF books that feature chlorine breathers (James White's Sector General series, for example) which is why I think they included the chlorine atmosphere in SE3.