The paper itself says "The phenomenon of biological ultraweak photon emission (UPE), that is, extremely low-intensity emission (10–103 photons cm–2 s–1) in the spectral range of 200–1000 nm, has been observed in all living systems that have been examined." Human vision falls within that spectral range, but the brightness is several orders magnitude too faint to see with the naked eye, even in absolute darkness.