Ok, so just for example - if I steal your car, hide it so no one can find it, go to prison for a year or two, get out.....the car is mine? Or a phone, or money, or pretty much literally anything else? A stolen property remains stolen until it's returned to its owner, it doesn't transfer ownership just because the thief served time.
The judge will almost certainly order the return of the car (or a cash restitution) as part of the sentencing. If you don't return the car or don't pay the restitution you are in contempt of court.
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.