I haven't seen the video, but after reading your post I decided to conduct my own test to see what it was all about.
I grabbed a stack of 10 silver dollars to test with. I tested in 5 & 15kHz.
Mind you, this was also all air testing, which I don't put a lot of stock in. And IMO, fresh planted isn't much better than air.
One silver dollar gave the response we expect.
Two and three dollars stacked still sounded good.
After that things started to get a bit more interesting, and I saw 5kHz returning better tones than 15kHz, which didn't surprise me. Response started to vary depending on proximity to the coil. Close it started wrapping around, but a few inches away it still sounded good. As many as seven stacked up did about the same, but I needed to get progressively further away from the stack for a good response. By the time I got to all ten in the stack I had to back off to nearly the maximum detection distance or it would wrap around and sound like junk.
My conclusion is that it has to do with total mass combined with the conductivity of that mass. And when I say it sounded like junk, I mean that it was a mix of tones that we typically would attribute to trash. If I were in a park and I got tones that sounded like that, I would keep walking. But, If I were in a field like where I like to hunt, I'd be digging it thinking it might be something good mixed with iron debris.