She was 12 years old when Krypton blew up, and she had traveled extensively offworld. If there were an existing Kryptonian expatriate population, she would certainly know of it, just as any 12-year-old American who'd frequently traveled abroad with her family would know that there are plenty of Americans living, working, and traveling in other countries. This is just everyday common sense.
What have human beings got to do with it? They're survivors of a dead planet. If there were others of their own people out there, they'd feel an obligation to find them and reunite them so Kryptonian civilization could survive, the same as Rhea feels obligated to reunite the Daxamite refugees -- and the same as Spock Prime felt obligated to help the surviving Vulcans in the Kelvinverse.
And once more, it doesn't matter how much we speculate about what the answer might be. The problem isn't that we can't think of an answer -- heck, explaining plot holes in an SFTV franchise is a large part of what I do for a living. The problem is that the show's own writers created this inconsistency and have failed to fix it, even though it wouldn't be hard for them to do so. If you ordered a pepperoni and sausage pizza and it came without sausage, you wouldn't defend the cooks' omission and try to cook up your own sausage to put on the pizza -- you'd complain that they made a mistake and demand that they fix it. And you'd be right to, because they're the ones selling the product and you're the consumer, and it's not your responsibility to excuse their shortcomings.