Walt Disney was born in Chicago. I know of Chicago. And even if he was born on the South Side, he wouldn't have a southern accent. He then moved to northern Missouri and soon after to Kansas City. In none of those place is a southern accent the dialect. And then, when a teenager, he moved back to Chicago for high school.
But even if one wanted to argue that, well, some of the Midwest states (Illinois and Missouri, included) have southern roots, there's a pesky problem -- we know what Walt Disney sounds like. He hosted a TV program for many years. He didn't have even close to a southern accent.
Maybe when one sees the movie, that little twinge of a southern accent disappears. Maybe Hanks only tosses it in to come across as chummy, in some way, and for most of the film he has the flat, Midwestern accent we know that Disney had. I have to suspect that's the case, he's too good and smart an actor to play Walt Disney with a southern accent all the way through. We'll see... But even those few sequences are odd.
But it still looks great.