The south on its own is not enough to win the nomination.
Ehh, I suppose that's true in a vacuum/strictly mathematical sense, but it's not really correct in how things tend to play out (namely, that candidates will be receiving delegates & votes from other places as well). Take Clinton in 2016: her 55-43 PV win would have become 51-47 if she had merely broken even in the core states of the Confederacy. She would have tied nationally just by breaking even in those states + MD, CA, NJ & NY (and I suspect given the inherently "packed" nature of black voters in urban areas, Sanders would have eked out the delegate win nationally in a tied PV scenario).
Given that there are still likely to be more candidates on the ballot when these states begin to vote (and therefore more outlets for protest/anti-Biden sentiment) than in 2016 and the fact that a candidate must hit 15% in a congressional district or statewide to receive delegates, it's not unfathomable to think that Biden could walk away with 90% or more of the delegates in a slew of Southern states (and something like 75-80% regionally; Clinton got a little over two-thirds by comparison) despite receiving a drastically smaller percentage of the vote.
If this poll was/is accurate, then Biden would be snagging every state-level delegate in SC, and probably all but a few at the congressional level -
with 33% of the vote. If a scenario like this plays out, then it's very possible that the difference between Biden winning or losing will be in the difference between breaking even in the South versus whatever share of the delegates he gets here.