If the gold was bright and shiny, I could see them getting caught. However, if you mixed in enough copper to give it a tarnished appearance, it would not look like gold.
There are not enough customs agents to check for every piece of metal with a special conductivity machine.
The smugglers are probably still doing metals smuggling with more concealable alloys. Customs would have to be tipped off to check them.
I don’t know how they were caught; it was a news story at the time (a couple of years ago). It might have had nothing at all to do with Customs.