Factual outcome:

• Joe Biden did secure enough delegates in the 2024 Democratic primaries and became the presumptive nominee, winning an overwhelming majority of pledged delegates. (en.wikipedia.org)
• After a widely criticized June 2024 debate and intense intra-party pressure, Biden withdrew from the 2024 race on July 21, 2024, becoming the first eligible incumbent since Lyndon B. Johnson in 1968 to end a reelection bid after locking up the delegates. (en.wikipedia.org)
• Following his withdrawal, Biden endorsed Kamala Harris, who was then chosen by the delegates and became the official Democratic nominee on August 5, 2024. (en.wikipedia.org)
• Harris, not Biden, was on the ballot as the Democratic nominee in November 2024. (en.wikipedia.org)

Interpretation vs. prediction:

• The normalized prediction: “Barring an extreme unforeseen event (e.g., major health crisis), Joe Biden will secure enough primary delegates and will be the official Democratic nominee for U.S. president in the 2024 election.”
• Reality: Biden did secure enough primary delegates, but he did not become the official nominee, because he later withdrew and declined the nomination.
• However, Sacks explicitly framed his forecast as conditional: “unless he changes his mind or there’s some act of God, he’s going to be the nominee.” Biden did change his mind and stepped aside in what contemporaneous coverage and later analysis treat as an unprecedented, extraordinary political event—an incumbent withdrawing after clinching the delegates under intense pressure from his own party. (en.wikipedia.org)

Because the prediction was explicitly limited to the world “barring an extreme unforeseen event / unless he changes his mind,” and the actual world is precisely one in which he changed his mind amid an extraordinary, historically rare chain of events, we can’t cleanly score the statement as right or wrong. The outcome falls outside the scenario he was conditioning on.

Hence, the fairest evaluation is ambiguous rather than definitively right or wrong.