Someone made it in the US, aged it, buried it in the manure pile for a few years.
That's one of the main points of the article. If treasure hunters simply pull artifacts out of the ground without careful recording of the exact location and other conditions of the area it was found in and put them on the market, it makes it extremely difficult to determine the authenticity of these artifacts.
It's actually very easy, if experts are allowed to examine it closely.....and yes, there are plenty of experts. They can look at the manufacture methods and take tiny samples of the metal to determine alloys.
Just let someone who makes replicas exam It closely.