I'm one of the scientists affiliated with this project. It's very true that SETI in general terms is a "needle in a haystack" sort of search. So one way to look at this is that we are suggesting more "needles" to look for. So far, we are searching for radio waves and optical pulses. Looking for technosignature molecules in a planetary atmosphere (if it actually works, which is what we are trying to figure out with our proposal), is a third needle.
I also totally agree with your points about looking for "inadvertent" CFC use by a young and dumb civilization like ourselves. As Hans Adler pointed out, we on the science team are thinking of this more as a search for terraformed worlds. For instance, if we were going to colonize Mars and try and live on the surface in the next century or so, we would likely need to warm up the atmosphere, and CFC's would be a good starting place (and possibly detectable over interstellar space)