Both “diet” and ”diabetic” beer are really the same and should not be categorised with low alcohol beers.
Low alcohol beers are basically normal beers with the alcohol percentage reduced.
Diet and diabetic beers must conform to regulations and the main requirement is that they contain no unfermented sugars, no matter what the alcohol level is.
In home brewing we assume that the fermentation (both primary and secondary) ferments all the sugars available; however, the yeast cells usually leave a small percentage just as you don’t eat all the crumbs on your plate.
Some higher sugars are unfermentable.
Special formulation and treatments are necessary to ensure that regulations are met, when breweries make diet beer.
Home brewed beers will always contain some minute quantities of unfermented sugars: however, many diabetics tell me that well matured home brews give them fewer problems that normal (non-diabetic) commercial beers.