It's never explicitly stated that Praedyth was a Guardian. Literally all we know of Praedyth is that "(his) fall isn't over" etc. and "He skipped like a stone on time's ocean. (Praedyth's Timepiece) is what's left." Atheon's Epilogue, Praetorian Foil, and Hezen Vengeance all directly reference Vex minds. Vision of Confluence and Fatebringer are left ambiguous and the other two weapons refer to Praedyth. It's entirely possible that Praedyth is another Vex mind or entity. It is, however, explicitly stated that Praedyth is male, so the Exo Stranger is [b]not[/b] Praedyth. Another theory suggests that Kabr became Atheon, which intrigues me at the very least. Vex cells assimilated Kabr, as is known; he became one of them, if he survived. The Oracles and Gorgons saw some reason in letting Kabr continue to exist. And lastly the word "atheon" translates to "godless"; compare to Kabr's title of "legionless", and the fact that the Vex worship the Heart of the Black Garden, but a Guardian turned Vex would have no real reason to. It's a stretch, but given the limited amount of clarity we've been offered, every theory is a stretch honestly. It goes on further to imply that Kabr used the timegates to fashion the Aegis, and ensure he could be defeated and the Vault conquered, and then the Mythoclast, which explains why it is fitted for human hands (which is questioned on its Grimoire card).