It's a marketing ploy by AT&T to label some kind of slightly better (maybe?) 4G as 5G, kinda like when phones started to say 4G which didn't mean 4G LTE.

AFAIK this is deceptive marketing from AT&T: https://www.businessinsider.com/iphones-on-att-now-show-5g-e...

It's not really 5G; just an AT&T marketing gimmick. https://www.tomsguide.com/us/5ge-explained,news-29329.html