Citadel Advisors has only held small stakes in TSLA. I believe it is an algo based hedge fund.
Citadel Securities is a TSLA options market maker, and it still holds >5% of Tesla, because it has not updated the 13G it filed earlier this year.
However, there is no way it will manipulate away the S&P inclusion. A market maker is not in the business of manipulating or making directional bets, it makes profits off of the spread and does its best to hedge away any delta exposure to the underlying stock.
Yes, Citadel could technically sell all its shares and have a large influence on S&P inclusion, but it won't, because it'd be screwing itself over. Citadel needs those shares to sell to DITM options holders when they exercise, and to stop itself from losing tens of billions of dollars the next time the stock moves up.