how does Magatsu know about uncle being from Japan too?
I might be but but, I think Magatsu just figured it out from Uncle having such over-powered abilities. Uncle knew practically right away that Magatsu was from Japan so it's a wonder it took Magatsu so long to figure out that Uncle was, despite having been around a lot longer.
Perhaps Uncle is genuinely the first Isekai-ed person, other than themselves, Magatsu has met though. It seems to be quite rare; we only know if 4 in the manga so far and all apart from Uncle have actually been dead (Mabel's sword owning ancestor, the hotspring family's ancestor and Magatsu).