Joe Biden ultimately withdrew from the 2024 race and Kamala Harris became the Democratic nominee, so there was never an actual Biden–vs.–Trump general election with RFK Jr. on the ballot, meaning the precise scenario Chamath described did not occur. (arxiv.org)

Empirical evidence about where Kennedy’s support came from is mixed and time‑dependent:

• Spring 2024 polling summarized by Brookings (drawing on Quinnipiac and NBC data) found RFK Jr. pulling a larger share of Trump than Biden voters in a three‑way test (e.g., 12% of Trump supporters vs. 7% of Biden supporters in one Quinnipiac poll), implying at that point he hurt Trump more. (brookings.edu)
• By July 2024, a Pew survey showed that among Kennedy supporters, 47% said they would choose Biden and 44% Trump in a two‑way race, suggesting that if Kennedy were removed from the ballot, slightly more of his then‑supporters would flow to Biden than to Trump—which in turn implies that his candidacy at that moment was marginally more damaging to Biden. (pewresearch.org)
• After Biden withdrew and Harris replaced him, Pew’s follow‑up found that Kennedy’s July supporters who switched candidates broke 39% for Harris versus 20% for Trump, indicating many of his earlier backers were at least somewhat inclined toward the Democratic ticket. (pewresearch.org)

At the same time, Kennedy suspended his campaign in August 2024 and endorsed Trump, but remained on the ballot in some states. (pewresearch.org) In the final results he received about 0.49% of the national popular vote, and Trump’s winning margins in closely watched states like Wisconsin and Michigan exceeded Kennedy’s vote totals there, making it difficult to show that his lingering candidacy clearly hurt one side more in the actual outcome. (en.wikipedia.org)

Analysts also disagreed ex ante: Democratic‑aligned Third Way argued Kennedy was disproportionately attracting disaffected 2020 Biden voters and key Biden‑leaning groups (young voters, moderates, independents), warning he could be a spoiler against Biden, while the Brookings review of multi‑candidate polling concluded the combined third‑party effect on a Biden–Trump race was near zero because Kennedy’s pull from Trump was offset by Stein/West drawing more from Biden. (thirdway.org)

Because (1) the specific Biden–vs.–Trump general election never actually happened, (2) high‑quality surveys at different times point in different directions about whether Kennedy drew more from Biden’s or Trump’s potential voters, and (3) his truncated, small‑vote candidacy in November makes his realized electoral impact hard to quantify, there is no clear, settled empirical answer that his independent run definitively hurt Biden more than Trump. The available evidence allows arguments both ways, so the prediction cannot be confidently scored as right or wrong and is best labeled ambiguous.