Very interesting, thank you! I especially like the insight that for evolution to go on uninterrupted for 5 billion years, you don't just need a particular type of planet (not hot, not cold, in a stable orbit) in a particular region of the galaxy (on the outskirts), but this planet also needs to be inside a particular type of galaxy (big, old, not dense) that happens to be in a particular type of intergalactic environment (not dense, lacking low metallicity dwarf galaxy neighbors). This helps with the sharper version of the Fermi paradox that assumes the possibility of intergalactic travel.

I'm not a physicist, but as far as I understand the paper, their assumption of what constitutes a "lethal" amount of Gamma Ray Burst damage to a planet seems kind of arbitrary. Their description indicates that it'd kill everything on the surface and everything underwater that feeds on plankton. But I see nothing to indicate that, say, life on hydrothermal vents, or bacteria living deep underground (which exist on our planet at least two miles down) couldn't survive what the authors call "lethal". So abiogenesis would not need to happen again in those cases, nor would evolution of very basic metabolic structures that evolution would again build upon. Even a small bunch of tiny replicators that survived with, say, three billion years of previous evolution under their belt might re-colonize the planet much more quickly and diversely than newly arisen ones could.

Meaning that as a layman, I don't see how we'd distinguish between a past where Earth was hit by a "lethal" GRB 2 billion years ago (when there were just eukaryotes, procaryotes and cyanobacteria), and one were it wasn't.