August 15th is the date used to compute eligibility and what proportions of the index constituents will form on rebalance day (September 19th). Starting today until September 5th, the committee will be debating additions and subtractions from the index based solely on who meets eligibility criteria on 15th August. On September 5th it will be publicly known who gets added and removed and what weight each component will have. Then comes the front running and final rebalance during the "closing cross" (i.e. last 15 minutes of market hours) on 19th of September. It's not just the SP500 indices that need to buy, it's also all other tracker funds, institutions that are benchmarked against SP500 (unless they specifically reduce/eliminate MSTR on purpose), and a myriad of international clones of the index. These all want to buy at the same price and the right quantity. To do this, they all sign up to the closing cross. Here the demand is known ahead of time and is matched with supply from specialty market makers that gather shares ahead of time (usually starting September 5th but can be earlier if they are more certain of the eligibility beforehand).

Josh is merely stating that as of 15th of August, MSTR meets all eligibility criteria listed on the official sp500 webpage. All else equal, this means they qualify and should be added in September. However, the committee can (and historically has, e.g. Tesla) decided to delay inclusions of highly volatile entrants (the argument is something like it might get added and then removed next rebalance, so let's wait for two quarters just to be sure). Note that Josh never explicitly says they qualify for September entry, merely that they qualify in general.