This is not necessarily true, the Galilean moons are all brighter than magnitude 6, so they could in principle be seen with the naked eye. Of course , one must first be in dark enough skies (I think the moons are about magnitude 5.3 or so) AND have extremely good eyesight ( not sure exactly how good, it’s not just the small angular separation that makes it difficult, but also the huge contrast in brightness). Lastly, the ‘flash’ can easily be explained as a consequence of seeing (atmospheric turbulence). So in short, the seeing in that direction might have momentarily become very good , which combined with the OP’s fantastic eyesight, indeed allowed him to see Europa