It's just not true. Even investing in treasury bonds today has risk of loss.
There is no such think as risk-free investments. It can be risk-favored, for sure. And it is definitely a benefit to the receiver. But its not a "guaranteed killing".
This conversation is about a hypothetical where we have a trillion dollar loan with no interest. In that situation, taking on the lowest risk for the lowest return, or "bank checking account" would still make you rich. You wouldn't need an amazing knowledge of investments.
In actuality, trying to invest that much independently would probably damage the economy, and it's difficult to have any clue what would happen.