If you look closely at Bennett's skin, you can see that his skin is slightly darker than the rest of the inhabitants of Mondstadt and if he is from Natlan, then he is probably a half-breed. And my theory is that Iansan's mother started an affair, let's say with a diplomat from Fatui, and Bennett appeared and because the child was born before marriage, Bennett's biological father decided to hide him at the edge of the world and took him to a dungeon where an adventurer found him.