Two separate issues make this prediction impossible to score as right or wrong at this time:

1. The conditional scenario did not fully occur. Joe Biden initially ran for re‑election, but he withdrew from the 2024 race on July 21, 2024, after securing the primary delegates, and endorsed Kamala Harris, who then became the Democratic nominee. Donald Trump became the Republican nominee and went on to win the 2024 election, being inaugurated on January 20, 2025. (en.wikipedia.org)
   The prediction was explicitly conditional on both being candidates in 2024; in the realized timeline Biden did not remain the Democratic candidate into the general election, so the comparison Chamath was describing is now partly counterfactual.

2. The forecast horizon (a full 2025–2029 term) has not elapsed. As of November 30, 2025, Donald Trump is serving as president and is publicly active (e.g., pursuing his policy agenda and even musing about 2028), which indicates he is presently functioning in office. (theguardian.com) However, the claim was about which man had a higher probability of remaining physically and cognitively capable of serving out an entire four‑year term starting in January 2025. Trump’s current term runs until January 20, 2029, so whether he will remain fully capable for that entire period is not yet knowable.

Because (a) the condition about both being candidates did not hold in the way implied, and (b) the full four‑year period has not yet passed, the prediction cannot currently be evaluated as correct or incorrect, so it is inconclusive (too early to tell).