No, stolen property does not become unstolen with the passage of time. If someone steals your car, and hides it in a storage locker for 5 years, they don't get to keep it. The SOL does not begin to run until the facts giving rise to the cause of action are known. In this case, if the US found out that this family had the coins, and did nothing about it for 5 years, or whatever the SOL is, then the statute ran out. But if an employee stole the coins, hid them in his closet, and then they passed to his grandchildren, the SOL would have "tolled" all that time, until the facts that allowed the rightful owner the ability and right to sue for their return were known.
Good comment.