The first rule of treasure hunting club is don’t talk about treasure hunting. This duo is learning a hard lesson. If you find a target that is too large to recover on the sneak just let it go and move on. At least the Feds won’t get it.
Most states require that anything found be turned over to Law Enforcement for safekeeping until the legal owners or their heirs can be located.
If the find was at any time owned by the government you are required by law to notify the government. Plus, IIRC, this gold was supposedly on Federal land. In that case you are required to notify the government before you begin digging.
In the case of a treasure trove, a large amount of precious metals, coins, paper money or art objects, the finder is required to notify the government who will take possession of the find.
The government is supposed to pay a finders fee for any treasure found on Federal land or which once was in government possession.
Only about five states have a Finders Keepers law.
IMHO the FIBbies hid the find and kept any finders fees for themselves.