I can’t imagine how they’d get reliable spectroscopic information on something that small, faint, and distant. Imagine something the size of the Earth orbiting Proxima Centauri (the closest star to our sun). I doubt that they’d be able to distinguish it from (say) something like Venus.
It would be speculation at best. Since chlorophyll absorbs light (except for green 500-600nm) saying that astronomers have 'found' chlorophyll means that they simply have found nothing in that spectrum. It would have to be comparitive spectrometry between reflected red and blue wavelengths (<500nm annd >500nm) but that could be practically anything without direct observation.
Funding! That’s how they do it. And then they’ll need motivating results for more funding, so the results will be “encouraging”. By the time the fundees die, they’ll have made a good life on scamming Uncle Sugar. Let’s just be sure none of these failed AGW scientists didn’t move over to this project.