Wrong for two reasons:
1. There is no restriction that the asset being borrowed and the collateral are in the same asset class - loans can be extended based on whatever criteria the DAO in charge of the Dai votes on. There are a number of loans backed by real property investments in the US, see [0] under RWA. There are a number of loans also backed by large holdings in US bonds.
2. Dai is pegged to the dollar. It can be instantly exchanged with the dollar at a number of onramps. It is not equivalent to holding volatile crypto like ethereum.
[0]: daistats.com
