You are wrong. In most jurisdictions theft is defined as taking something away from the legal owner. If what you said was true then taking away your own property from the thief who stole it would also be a theft.
No, that's now the scenario the parent poster gave: They never even suggested that the Wallet-Taker was somehow the true legal owner of the cash!
All they said is that the other person, Wallet-Dropper, is something less than the prima facie legal owner. This permits several possibilities, but in every case Wallet-Taker still seems to be doing something wrong:
1. If Wallet-Dropper is a thief, then Wallet-Taker is just a thief stealing from a thief.
2. If Wallet-Dropper is the well-meaning temporary custodian of unclaimed property that they cannot (yet) claim as their own, then Wallet-Taker is a thief, taking to deprive the legal owner (wherever they are) of their property.
3. If Wallet-Dropper is the new legal owner of the discovered cash, that's theft by Wallet-Taker no matter how much you think Wallet-Dropper "didn't really earn it."