It's reasonable to be skeptical, but it is even more reasonable to evaluate a peer-reviewed publication in one of the most rigorous subfields of psychology (sensation & perception) after having read at least its abstract, ideally the whole paper.
"Landmark experiments by Hecht and colleagues in the 1940s established that dark-adapted human subjects are capable of reporting light signals as low as a few photons (~5–7)."
Looking around, http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.1979.sp0... is a 1979 paper showing that a toad retina can respond to a single photon.
The claim from this new paper doesn't seem that extraordinary given what's been known for decades.