1. The authors note that the lunar surface partially depolarizes the light. Wouldn't this happen much more with light coming from very far that has hit multiple potentially depolarizing surfaces? Light could also be depolarized by dense atmospheres or by interstellar media like dust grains and ice grains. More interestingly, the polarization could also be reversed or affected by chiral compounds in outer space.
2. A related question: how intense does the light have to be when it reaches the detectors? Presumably light from worlds that are billions of light years away is going to strongly interact with surfaces and interstellar media and lose most of its intensity.
3. It's clear that chlorophyll is responsible for the signature of vegetation. Alien plants may not necessarily utilize chlorophyll as the light harvesting pigment, in fact they may well be equipped to use alternative wavelengths. There could also be life not dependent on sunlight. How we will be able to interpret signatures arising from other unknown pigments and constituents of life is an open question.
4. It is likely that advanced civilizations have discovered this method of detecting life. Could they be deliberately broadcasting polarized light to signal their presence? In the spirit of a past post, could they do this with specific molecules like amino acids, isotopically labeled molecules or stereoisomers? How sensitive is the polarization to molecular concentration? Any of these compounds would strongly suggest the presence of intelligent life which has developed the technology for the synthesis and purification of organic molecules.
Sterzik, M., Bagnulo, S., & Palle, E. (2012). Biosignatures as revealed by spectropolarimetry of Earthshine Nature, 483 (7387), 64-66 DOI: 10.1038/nature10778