Keep in mind that 4 billion (likely Softbank invested more) means 100x in stocks. i.e. a 10 billion investment in call options 'generates' 1 trillion in extra market cap (because of dealers hedging). That is 1/3 of AMZN or 2x TSLA and so on at current (not March) valuations. Such concentrated volume CAN move big stocks and in turn indexes.

Even if the market let go a bit now - Softbank has literally cornered the market for so long - they still make a killing in the process.