Pretty simple, really.
About $234B of capital* flowed into TSLA between the S&P announcement (Nov 16) and the Close on the freeze date (Dec 9th).
Recent estimates are that about $80B in TSLA equity will need to be transfered to Index Funds. That's about 34% of the capital that's already moved.
So,
do you think fewer than 1 out of 3 shares purchased over the past month will become available through the market for Index funds to buy?
TL;dr Yes, it's quite possible that there is enough liquidity in the system to avoid a large SP spike. That's likely what the S&P meant when they said this in their Nov 30 announcement:
"In its decision, S&P DJI considered the wide range of responses it received, as well as, among other factors, the expected liquidity of Tesla and the market’s ability to accommodate significant trading volumes on this date."
*
Note: yes, shares issued by Tesla on Dec 11 for the the $5B cap raise
will be counted in TSLA weight for the S&P 500