Actively managed funds have choices in what they invest, and how much they put into any of the companies. Index funds do not.
https://www.etf.com/SPY#overview
for an example.
The rest is chicken little - we got into that mess before because neither CDOs nor CDSs were liquid which means that as long as they were not trading banks were able to continue book them at the nominal value and since to trade them one needed to have banks that were on a hook for them to do the trade nothing was moving. That's the biggest issue with those kinds of instruments.
Nothing of sort could happen with the index funds because both the derivatives and the underlying are sufficiently liquid, so the drop won't be 100->20 but rather 100->99.9->99.8->99.7->....->20.1->20 which in turn would re-balance everything.
What eventually happens is that there's not enough new money coming in for index fund buy orders to maintain the bottom of the underlying prices, which means that the prices start drifting lower, the popular financial press goes nuts, CNBC/FoxBusiness/WallSt Journal/etc blast it in the headlines and the same way goes the other way. People start selling their index funds, which makes the index funds sell the underlying which creates a massive wave of the sell orders creating a downward pressure, which in turn puts brings the indexes lower which makes press go screaming more which causes more people to want to "protect their test egg" by selling the index funds they had.