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.
I mean sure, and tbf he may well come in second in NH (we'll see), but at least right now he's probably looking at a third or even fourth place finish in Iowa. Obviously, a lot can happen, but I could see a distant third or fourth in Iowa costing him a lot of support from folks who assume he's the only electable candidate and if he then underperforms in NH too (also possible), I don't know that he can recover from the narratives that's gonna create.