I really don’t think it is a white supremacist history. River city iowa is a fictional town but it’s probably based on Willson's hometown of Mason City, Iowa. Current demographics is 93% white. I can only assume that it was even higher in the 50s or when he grew up (he was born in 1902)
If perhaps we replace the word "supremacist" with "privileged," I think you might see the rest of your statement here a bit differently.
In other words, we do have to face the fact that we have a huge and wonderful legacy of musicals that nevertheless show a willful de facto white privilege world. I'm not blaming this on Willson in this case, but this is still part of the history of American musicals, like it or not. Those of us (especially Caucasians) who grew up with these shows may not have thought of them that way before - but we are being challenged to see them that way now.