Facts don't bare that out. Berne won the Iowa caucusus, big deal, then NH and union NV, that was about it until he edged him out in CA, Biden pretty much ran the table after that not counting VT, Utah and some other small states
But that was the pivotal point. Biden looked as listless and clueless as possible. Kamala was calling him a racist on the debate stage, and he had no response.
When Clyburn endorsed Biden, he came out and said he wasn’t happy with the way Biden’s campaign was running and expected him to make some changes to turn it around. If Clyburn had endorsed Sanders who was already 3-0 going into SC (or anyone other than Biden), Biden is done.