I agree. It depends on the value. If it's a wallet with an ID and CCs. Most likely the owner will at first chance cancel all CCs and request new ID. So the value is in the wallet itself. For the most part billfolds are cheap.
If it has lots of money that amount probably is a non-trivial amount to the wallet owner and you feel obligated to return it as you would want the same.
Looks like they didn't adjust for PPP when they did the experiment. Not sure it would make _much_ difference. But $13 might mean more in some places than in others. Even within the US. $13 in San Francisco vs $13 in dusty Fresno.