This is a great question and has finally lured me out of my status as lurker on this excellent forum. I was born and raised in England and have spent the last half century or so in the US, so I am familiar with the many varieties of both "biscuits" in the American usage ("biscuit" means "twice baked" in its original Latin configuration. It does not rise and is a "cookie" in the US.) American biscuits, made of flour, baking powder, and fat are close relatives of the English dumpling in composition, but not in cooking method. All dumplings that I can think of, with the exception of the baked apple dumpling, are cooked in stock or saline. Popular European examples include potato dumplings and the tiny spaetzle from Germany and the pasta based gnocchi from Italy. English dumplings use ingredients that are almost identical to the U.S. Southern biscuit, except that they traditionally use suet, cow organ fat, instead of lard, which is pig organ fat.
The seasoned "lumps" (origin of "dumpling" are dropped into beef stew, where they sit totally submerged and double in size from the action of the baking powder. Yum.
I first saw the biscuit topping mentioned in an English cook book by Anne Willan (Chicken Classics, 1992), and when I was living in San Diego about 15 years ago, it covered what was laughingly called "steak and kidney pie."
I consider it inferior to a pastry topping in both flavor and consistency and it is certainly not a dumpling, but when i cook for my culinarily challenged daughter (but not her daughter, who, at eighteen, is a gifted cook) I will serve pot pies in ramekins with this "biscuit topping" fancied up with a few mild spices and some parsley, and it is quickly made and satisfying.
Cheers