I don't know about "most", but it is probably safe to say "many." Sallie Mae retains ownership of 25% of all outstanding student loan debt. If the debt was forgiven, I think it is safe to presume their equity stock price would go to 0. Look here for the largest holders of that stock:

https://finance.yahoo.com/q/mh?s=SLM+Major+Holders

Much of the other 75% is I'm sure also distributed into many other common mutual funds that are used in people's 401k plans.

>Do most 401Ks really invest in student debt?

It's trillions of dollars, and every penny of it is held as an asset on someone's balance sheet. Do you think it's confined to "greedy bankers"?

They might; I doubt they do directly. The emphasis is on the "interconnectedness". After five or so hops ( A invests in B ->C->D->E ), you don't much know any more.