Per the S&P 500 website, the claimed subset of the market is "U.S. large-cap equities". S&P claims their index is "best single gauge of U.S. large-cap equities". But, it's clear that given the current iteration of the rules, none of the major upcoming IPOs of Spacex, Anthropic, or OpenAI are eligible for S&P500 inclusion, and they likely will not be for years.

Claiming to have the "best single gauge of U.S. large-cap equities", yet having rules that exclude three of the top 20 largest U.S large-cap equities which make up ~5% of the total market cap of the U.S. stock market, means your benchmark is inaccurate by my book.