A simple drill would discover the fraud.
Ever see the cartoon (I think it was the Underdog opening titles) where he bites the coin? I think that was an old habit to test if a coin was gold and soft to the tooth, or fake.
My read is that it has never been alleged that anything other than 400 oz. bars have been the subject of Tungsten fraud. Coins are not a factor, although lead fakes are well-known, and easily detected with a scale and a caliper.
This would make an interesting plot element in a novel, however. Presidents funding their corrupt future by defrauding Ft. Knox ad the Chinese. James bond-level of clever field detection of fakery. Etc.
The giveaway for me that the story as written is bogus is the "certain detail" (sic) concerning 400 oz tungsten blanks, being plated in a paper thin layer of gold. No way. The fraud would be too easy to discover, even by humble vault employees during normal stacking and counting.
If this story said that 300 oz tungsten blanks were molded inside of 100 oz of gold, it would be more plausible. Then, fraud between willing players might work. (But both sides would have to be aware of the fraud, and have a reason to keep quiet about it.)
But a 400 oz tungsten blank with a few mills of gold plate?
No frikkin way.