As I understand it ... and at this point I think I'll set aside picking on Snyder for MORE chosen one tropes ... this is J'onn coming to terms with the fact that "Cosmic Forces" are at play in his timeline and backstory and trying to point out to Lex that those same cosmic forces are mucking about with his as well - the awareness of some shared historical "forgotten moment" coming to them only now, even as the Perpetua Crisis looms not even meant to be ironic or a retcon to make a story melodramatic, but part of the story structure itself.
Young J'onn was not just abducted from Mars, he was abducted from TIME as well. He's time-displaced thousands of years as he's poked and prodded by the Legionnaires Club. It's another cosmic manifestation of the same "there's a conspiracy behind everything" beat that Scott Snyder began with, with the Court of Owls ... that became various Metals ... that became Metal ... that became Element X & Source stuff ... and is now going full-tilt Multiversal. (Actually once you factor in the Barbatos of it all, and the time travel that had happened during Final Crisis ... the Court of Owls almost make MORE sense as a ... well, a Retcon Conspiracy. A Retconspiracy. Which is unoriginal, sure, that's what Doctor Hurt turned out to be all along, too. But the notion that Cosmic Forces are actually subjecting various players in the DCU to Retconspiracies to line them up for Cosmic Crisis Destinies is not exactly hard to fathom in a universe made of narratives and shared-universe storytelling.
But you see this with Time-Travel storytelling all the time. Doctor Who essentially makes it into a yearly beat - some Time-Conspiracy ghosting the Doctor until the last episode of the season when we find out that it was HIM all along when he visited some place. Comics just have the ... magician's-trick distraction of being able to throw like 200 characters you care about at you so you don't necessarily see the play coming.
Perpetua's ... what, Nix Uotan's grandmother, then, right? Of a sort? Or great-grandmother, if the story-infection meant that Zillo Valla was Nix's "mother" in some way? Monitor-Kind ancestry is probably a horrible rabbit hole to go down.
I guess to end I'll say that Lex is still just Lex. Getting caught in the Retconspiracy cosmic tide for the whims of an event-level storyline don't change you in the long run. It happens to Superman like every other week!