You can see in the pics that the layer of gold is quite thick. I doubt the bars would get far if they could be detected so easily.
They know such checks are going to be done. They know gold plating will not work. That’s why they go to the trouble of hollowing out actual gold bars.
I’m actually far more interested in how they are hollowing out the actual gold bars, and how they are sealing them up again without this being visually noticeable.
What makes you think they "hollowed out" the gold bars in the first place. Far simpler just to make a cast of the gold bar, make a mold, melt the gold, and pour it in over the tungsten bars. No need to "bore" anything.