Chamath predicted that on election night 2020 there would be no final or reliable overall election result because Pennsylvania—and specifically Philadelphia—would not be fully counted or called, making any result that was called that night incomplete.

What actually happened:

• As of the early hours of November 4, 2020 (the morning after election day), no winner had been declared in the presidential race; key battleground states including Pennsylvania were still counting large numbers of ballots and were explicitly described as unresolved. (pbs.org)
• The Associated Press and major TV networks did not call Pennsylvania for Joe Biden until Saturday, November 7, around late morning Eastern time, four days after Election Day. That call, based on Pennsylvania’s 20 electoral votes, is what pushed Biden over 270 and allowed those outlets to project him as the winner of the election. (washingtonpost.com)
• Philadelphia’s mail-ballot counting continued for days after Election Day; thousands of mail ballots were still being processed after election night, and city officials emphasized that counting would go on around the clock until finished. Philadelphia did not finish counting and certify its results until days later. (whyy.org)

No reputable national decision desk or major network called either Pennsylvania or the overall presidential race on election night itself; the eventual calls came only after additional days of counting in Pennsylvania, heavily driven by outstanding votes in places like Philadelphia. (pbs.org)

Because (1) there was no final or broadly accepted overall result on election night, and (2) this was indeed due in significant part to Pennsylvania—and especially Philadelphia—still counting votes and not being ready to be called, Chamath’s prediction matches the subsequent events.