Being added to the S&P 500 does not prevent price discovery in the stock market. There are still enough active funds and active traders that stock prices can go

Being added to the S&P 500 does not prevent price discovery in the stock market. There are still enough active funds and active traders that stock prices can go up and down for stocks within the S&P 500, and sometimes stocks fall out of the index if they fall enough. Doubt that will happen with tesla anytime soon though unless there are some really bad announcements for their business or future.