They don't have to buy their own stock on the market. They can just not sell it in the first place.
If the stock drops in price, the value of the stock they owe will also drop in price. So, in that scenario they will not have $900B of commitments. But it's the same amount of stock.
If I own 10 apples and owe you 5, it doesn't matter if tomorrow apples cost $50 Trillion then 50 cents the next. I still owe you 5 apples either way, and I have plenty of apples to cover that debt to you regardless of how much the grocery store is selling them for.
Side note: plenty of IPOs are terrible investments a lose everyone money. They are risky investments.