The index fund is holding the underlying stock, largely. The challenge is that if say 5% of their fund holders sell their shares in the fund, the fund has to sell the underlying stock to generate the cash to pay out. Most funds have a rule in their documents that if you are a large fundholder (holding 1% or more of the fund) and you sell, they can hand you stocks directly rather than selling them and giving you cash, but that doesn't work it it's tens of thousands of small fund holders selling.
So? They sell others buy. If I hear there is a squeeze and have cash I'll throw it in: who wouldn't? It's never the case that x٪ of the market can exit at once regardless of how it is owned.
