That is the justification for the much maligned forfeiture laws. Without them, or some other lawsuit, yes they are his.
More so, "possession of stolen property" is also a crime. The car doesn't magically become unstolen just because the theft went to jail.
This has absolutely nothing to do with civil forfeiture.
Civil forfeiture is when the getaway car is seized when you are robbing a bank.
Restitution is when you are ordered by a court to repay the victim for their losses that your criminal behavior caused. This case involved restitution, if he didn't comply with giving up the private keys he would be ordered to pay restitution.
Civil forfeiture doesn't impact restitution.