The south on its own is not enough to win the nomination.
While that's true, it's not Biden is getting obliterated elsewhere either. If he comes second in IA and NH (which seems like a plausible outcome given the polls) and wins Nevada and SC he'd be a clear frontrunner. For example.
Biden winning Nevada after coming second in both IA and NH (unless he loses them to different candidates, I suppose, but that also seems unlikely) is implausible. At this point, Biden coming second in IA also looks implausible, although who knows.