"Mint officials asked to authenticate the coins, then confiscated them after doing so, Berke said."
****
There are highly-reputable professional coin grading services that could have graded and authenticated the coins and the coins would have been returned to the owner. The big mistake was in allowing a government agency to this.
The government hates competition.
they should have only given up one coin to authentic. Why would they trust the mint with all the coins or anyone else, surely they knew some of the coin's history before turning them over.
I recall reading some stuff about this type of gold coin in the past; IIRC, they are indeed illegal to be in private hands and couldn't be there legitimately. I assume a "reputable" grading service would at the least have notified whoever brought them to the service, if not the gubmint - which could well be the case.