Yup, I saw that too, made me instantly super-suspicious that coin was planted as a carrot for the proverbial mule... for funding the next season purposes. I mean, if they were going to pull real treasure out of there, why not a nice chunky gold or silver coin instead of a chintzy copper one?
I thought the first signals they got were probably old beer cans sloshing around in that swamp muck myself, seems like the ones I hit here at the lake where it's mushy ground come up as a pretty good sounding signal in the silver range, and even my little F2 can find them at about 18" or so, but they do seem to "move" around a lot for whatever reason.
I did get a kick out of the diver wearing a ring while using a pinpointer, that was pretty funny and is one of the reasons I don't wear my rings while detecting aside from not wanting to lose them, cause I've done that myself too.