The prediction was explicitly conditional: Sacks said that if the 2024 election were Biden vs. Trump, it would be a nail-biter/toss‑up with no clear favorite.

That condition never occurred. President Joe Biden withdrew from the 2024 race on July 21, 2024 and endorsed Vice President Kamala Harris, who became the Democratic nominee. The actual general‑election matchup was Donald Trump vs. Kamala Harris, not Biden vs. Trump. (nypost.com) Because the hypothesized Biden–Trump contest never happened, we can’t directly test how close it would have been “heading into the election.”

We do have polling data from when Biden was still running. In late June–July 2024, national polling averages and aggregators generally showed Trump leading Biden by about 2–4 percentage points nationally, e.g. RealClearPolitics, The Hill, and other averages had Trump up roughly 3 points at the time Biden exited. (en.wikipedia.org) Some modelers, such as Nate Silver, explicitly described Trump as a clear favorite rather than a toss‑up, putting Trump’s win probability around 65–66% vs. Biden. (nypost.com) That evidence suggests the race at that time was somewhat close in the numbers but not universally regarded as an even “toss‑up.”

After Biden withdrew, the race became Trump vs. Harris, and many analysts did characterize that contest as extremely close—Harry Enten, for example, called the 2024 election the closest presidential race in at least 60 years based on Harris–Trump polling. (vanityfair.com) But that pertains to a different matchup than the one Sacks specified.

Because the specific Biden–Trump general election Sacks was talking about never actually occurred, and we can’t observe how that hypothetical race would have looked right before Election Day, the truth of his conditional prediction cannot be definitively determined even though the 2024 cycle is over.

Hence the assessment: ambiguous.