I mean, this is just false. He's essentially the last remnant of the old Southern Democrats that have since been nearly entirely replaced by Republicans, but they absolutely did exist. Hell, god damn Missouri had a Democrat Senator and Governor not that long ago. Louisiana still has a Democrat Governor. The party used to have a pretty substantial conservative wing that was actually competitive in the South. They clashed with the rest of the party constantly, but they also played a key part in allowing major Democratic victories like the ACA to pass.
Manchin isn't that. He's a neoliberal. Southern Democrats were the precursors to neoliberals but manchin isn't one.
Southern Democrats were Democrats who voted against the civil rights act. Manchin isn't that old. He's been in office since the mid 80s. He is a classic neoliberal. Liberal on social policies, conservative fiscal policies wrapped in a feel good liberal gild.