The Client didn't really have much of a presence on that planet. Gideon certainly did, but he didn't come until well after, so I would argue that it wasn't terribly coincidental at all. And that if Mando didn't have the Child, even with that display they would probably still be alive.
Din didn't question that a Mandalorian would be alone on Tatooine. He simply questioned that he didn't already know of a Mandalorian on Tatooine despite spending time there.