Yeah, I don't really buy it either as the evidence is weak. The author does provide some references to some astronomers and physicists hypothesizing about past "immense solar outbursts" from the Sun, orders of magnitude more powerful than modern events but no specific observations or data are cited.

There is also some discussion of "recurrent novae," faint and fast "dwarf novae," and simulations of triggering mechanisms that could produce nova-level energy releases from stars interacting with phenomena like interstellar dust clouds but I'm not sure if the scales of any of those match what the ancient peoples described or if the simulations results are even plausible.

Still an interesting theory imo.