Are these the feral horses that are on Cumberland Island in Georgia?
I can't say definitively whether they are or aren't.
Cumberland Island had different horses introduced over time by the different people attempted to settle there (successfully and unsuccessfully). First by the Spanish and 200 years later after the English arrived.
It's would likely be almost certain that they have Marsh Tacky blood in them, but whether it's enough that the ALBC recognizes them as such I wouldn't know. Genetics testing would tell for sure. I've heard that the Cumberland horses have some behavioral differences from most Marsh Tacky, but that's hearsay and I don't put much stock in hearsay (you never know if there's an agenda behind the hearsay). I've also heard it said that they are Marsh Tacky. Genetics don't lie though, so that would be the real test. I know the CMTA apparently required genetic proof originally. Can't say if they still do. They might not for the offspring of already tested parents.
I know there use to be feral Marsh Tacky on Daufuskie Island which isn't far from Cumberland Island, but the ones there now are back to being domestic. I know a lot of people today think of them like the other east coast Spanish Colonial horses that survived on Islands, but the Marsh Tacky was actually not restricted to the coastal islands. They survived successfully in the marshes and swamps of the SC and GA low country. Were captured, used and traded by the native, and later European, inhabitants of the area.
It's interesting how quickly their numbers diminished between the 1950's and 2000, but it's to be expected since the uses for them were rapidly diminishing or disappearing.