5GE looks like 'ED'GE and the subscript E in the network marker even looks like what used to show for EDGE. So if your memory goes that far back it works, it's like 5G but with a bit of EDGE mixed in.
TL;DR: AT&T is putting an ad on your phone where the protocol marker normally goes. They are saying that their customers will be delighted to know that if they are in an area which, had they a different phone, they could be getting even faster service.