Assuming all complex life must bear certain chemical similarities to life on earth, which is a position I’m always skeptical about.
Ridiculous. Why assume photosynthesis is required?
Life is a thermodynamically dissipative structure, which means that it cannot persist without having both inputs and outputs at the same time. It gains the capacity to do work by transforming the oxidised inputs into the reduced outputs.
Without photosynthesis, the only available inputs are the stuff the planet already has on it, which is almost entirely already electronically reduced.
See the Wikipedia page on redox chemistry:
But there was life before the great oxidation event? I was taught oxygenic photosynthesis appeared whenever cyanobacteria showed up but there was life before that.
Lifeforms around deep sea hydrothermal vent is fully independent from photosynthesis. Even if the sun would have gone off, they would still have been able to sustain themselves for hundreds of millions of years. And the supposed ancestor to eucaryotes (e. g. Asgard archaea) have been discovered in probes from a deep vent. So the whole thing doesn't sound too convincing.
Although there's evidence for chemiosmotic methanogens using methane-originating electrons to boil water to turn an ATP synthase turbine as far back as the known Hadean rock inclusions in the Pilbara craton, the article specifies "complex life", not however defining it.
They leave that to the paper, which uses a fairly common standard for defining complex life: the development of segregated and access-controlled cell nuclei. There is literature from as far back as 2011 suggesting that the spindles necessary for nuclei to work cannot be maintained by an organism that can only get 2 ADP->ATP reactions per molecule of glucose burned and instead need a floor of about 11 to 14, but the method bacteria use to get 38 ATP per glucose does depend on photosynthesis.
Underlying paper[0]:
Dearth of Photosynthetically Active Radiation Suggests No Complex Life on Late M-Star Exoplanets
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