Assuming the dating of the olive tree fragment on the top of a hill above the caldera is correct, all it proves is that the tree may have survived beyond the generally accepted date of the eruption. By the way, the generally accepted date are supported by more than one artifact.
we know that olive trees are exceptionally hardy and live for a long time. Now if this artifact proved itself to antedate the accepted timeline via testing, that would perhaps move the date of the eruption back a few decades or more, but relying on one artifact for anything is unreliable in my opinion.
Exceptionally hardy doesn't mean it would survive the alleged supereruption.