All that the article seems to point out was that the Democrats' "Solid South" didn't flip overnight, which is obvious, and I've never seen anyone assert anything to the contrary. Of course some people who had voted Democrat their whole lives were reluctant to switch parties. But the change in the patterns of the electoral map could hardly be more stark, sudden, or complete in its transformation.
But that wasn't enough to lose the South to Democrats immediately, who after all had much better campaign and political infrastructure established there going back generations. So it's not true that the '64 Civil Rights Act and Nixon's Southern Strategy alone flipped the South, but it finished what already began decades earlier. And it was always about race.
Next you're going to deny that the Civil War was really about Slavery, right?