After Naofumi's group side reunites with Raphtalia's group and managed to return safe and sound to the castle, Kizuna proceeds to interrogate Glass about her attempts to invade other worlds to kill their Holy heroes and where did she got the idea from.
Glass says that after Kizuna disappeared and they looked all around for her, at a point the waves hit the world and they summoned their holy heroes, but upon reaching a certain point, the heroes refused to participate anymore on the waves, complaining that the waves had become too strong to handle and now they where waiting for "a patch update".
Glass had become desperate to protect the world, in hopes of finding Kizuna one day, so she researched everything she could about the waves and how to stop them. She discovered several old books that talked about the waves but one in particular talks about very very old legends about heroes invading other worlds, fighting against those worlds' heroes for survival, and of course kizuna is not fresh with this, questioning the validity about this legends, with Glass admiting that she did attempt the while hero-killing thing out of desperation.
So upon this, it wouldn't be strange that it worked similar with several other worlds, learning and recording about the waves on some format, usually reaching the point of "legends and myths" over time, this also means that any passed information could've suffered the ol' INFORMATION DEGRADATION.