Nothing really to do with having a TID or not but how you adjust the machine. The Tesoro is adjusted to only discriminate anything below iron. The added coin puts the target just above iron. The Alpha is discriminating out foil and below which would discriminate out the coin.
Here is a video I did with the DFX.
First and foremost anyone who relies on a visual ID around trash is definitely not working the site in a way to get all the targets, just the easier ones.
As for the test, the more a detector likes iron (which is a negative to me) the better it's probably going to do on that test. Plus it's flawed to begin with because you didn't scan the nail first to prove it wasn't it the detector wasn't seeing, but more importantly you can't mimic rusty nail conditions above ground.