I agree but the practical difficulties in selling $10 million in coins would be a problem.
How do you explain where they came from? The coin dealer will want to know.
How do you explain the sudden increase in your bank account? The IRS, bank, DEA, DHS will all want to know. They track that kind of stuff.
10 Million / $1300 per once (bullion value) = 7692.3 one once coins
I doubt they found 7000 coins. So, I am thinking the 10m figure is the expected numismatic value of the find.
Anyhow, to get the full numismatic value of the coins they had to go public and declare it to put it up for auction. Otherwise, they would have only gotten a much smaller amount. Even after taxes, they would probably still be ahead.