One of the places you could have learned this would be the article itself:

> most share indices weight firms in proportion to the value only of shares they have released for public trading (the “free float”). For SpaceX, this means just the $75bn or so of stock it intends to issue in June—so its initial weight in the S&P 500 will be around 0.1%. The NASDAQ 100 is an exception, and has changed its rules to weight companies at up to three times their free float, in an apparent effort to woo Mr Musk. Even so, SpaceX’s probable initial weight in this $40trn index will still only be around 0.5%.