While there isn't 125% available to buy in one go, there is trading happening, HedgeFund2 can be buying and returning their loaned shares this week, HedgeFund3 can be buying and returning theirs next week, HedgeFund4 the week after, and at no point does any group need to buy 125% of available shares all in one go; where does the time limit that causes the squeeze come from?