The one shot isn't unrelated! That little boy is not just coincidentally named Oriza. He's the actual young Oriza and his grandparents' (perceived) disappearance will come up again in the coming chapters. That's also why the mangaka says the story is a bit "anachronistic" because Oriza should have been a toddler in the 60s and not in the more modern times the one shot is set in.
The one shot isn't unrelated! That little boy is not just coincidentally named Oriza. He's the actual young Oriza and his grandparents' (perceived) disappearance will come up again in the coming chapters. That's also why the mangaka says the story is a bit "anachronistic" because Oriza should have been a toddler in the 60s and not in the more modern times the one shot is set in.