You seem to have ignored the 50% float rule. SpaceX is proposing to go public with about 5% of the float, but S&P requires 50%.
Do we think that the market will absorb the release of 45% of the shares? I'm dubious.
And while SpaceX is IPOing with 4% float, after the 6 months lockup many more shares will release and float will increase to 40-50%.
So after 12 months, SpaceX meets the S&P inclusion requirements.