They said the flare lasted 300,000 years, but how quickly does something like this start? How fast does it go from "nothing" to "full strength"? Also if there was any form of life orbiting a star in the path of this flair, is it safe to assume that said life is no longer alive? At least if that life resembles us in its tolerance (or lack thereof) for radiation.
Based upon a couple other articles I read [1] [2], it sounds unlikely there were other galaxies or smaller structures in the path of this flare. I only see references to a very directional cone at the poles of the Milky Way, and the cone only intersects a trail of gases left behind by the Magellanic Stream.
Were there somehow orphaned planets, solar systems or smaller galaxies in the path of those cones not mentioned in the articles however, then the gamma rays, X-rays and highly ionizing gases in those cones of energy would certainly strip away most if not all of the atmosphere of any Earth-like planet, and kill all forms of life as we know it unless it was in some kind of weird, tidally-locked configuration that kept one side always facing the originating, ejecting side of the cone.
These flares would make a great plot device setup for a story in the Xeelee Sequence or /r/HFY. We find out the flares are deployed as weapons, used as a kind of galactic-scale sawed-off shotgun against another galaxy. Or the flares are galactic-grade energy reactor experiments gone horribly awry, and are the cause of one Great Filter at the Kardashev Type-III scale.
[1] https://www.sciencealert.com/something-in-the-centre-of-our-...
[2] https://earthsky.org/space/explosion-milky-way-center-seyfer...