That's true for the S&P but not nasdaq, nasdaq is market cap weighted. There used to be a limit that the available float couldn't go below something like 20%. (This is because 5% float available but 100% market cap would cause a huge supply/demand mismatch). But for spacex they changed the rule so there's no minimum, it's just that below 20% float, companies would be weighted at 5x the float instead of 100% of market cap. If spacex is planning on something like 5% float, it would be weighted around 25% of market cap with only 5% of float available to buy.

But it gets worse because when the lock-up period expires in 180 days after ipo (currently scheduled right before quarterly index rebalancing), it's possible that frees up more than 20% of float and it suddenly has to be weighted at the full 100% of market cap -- triggering additional automatic buying.

It certainly seems like it's set up for our retirement accounts to be the insider's exit liquidity.