Yep. So, you don't just own the top 500 stocks by market cap. You own a set of stocks that resemble that idea, but are in fact still choices made by S&P. Also notice that stocks don't immediately get dropped when they fall below that criteria; there's a buffer for how bad they have to get to be dropped. Additionally, the rules governing these choices are free to change at any time. For example, whether stocks with split voting shares should qualify is still a discussion.

Let's also not forget Hacker News's favorite law: Goodhart's Law. The S&P has performed wonderfully well when it was observed as an index. But now that it's a target, the world will change around it.