Yeah, attackers aren't light bombers - the distinction here being that while in theory they can fight like dedicated light bombers, they can also do things light bombers can't, and the tactics of their use were adjusted accordingly. For all practical purposes, technically they are closer to twin engine fighter-bombers like Me 410, which happens to be very similar to A-20 in weight and gun armaments, while having a bomb load of 1000kg but somewhat better flight performance, than to light bombers of old.
Meanwhile dedicated light bomber Mosquito has even better flight performance and 1800kg maximum bombload, the main difference being total lack of forward or defensive guns, yet older, older and lighter than any of the above Bf 110 fighter-bomber could carry 2000kg of bombs in addition to few defensive machineguns and fairly strong forward guns.
So ignoring the inherent inconsistency of what different countries called them, seems like the closest things to decisive characteristics are the strength of forward facing guns, and implied tactics of the plane's use. The former makes a surprising amount of sense as it inherently affects the latter too - if you stick a whole lot of forward facing guns on a light bomber and plan to get any use out of them, how is it different from a fighter-bomber or attack plane?
After all that's what happened to Mosquito later in the war, and these models became fighter-bombers.
But Bf 110 did.
True, got confused about that one.
A-20 was always designated as "attack bomber", in USA at least, hence the A.
A-26, as its successor, was one too.
Only post-war, with the massive changes to air warfare happening around it, it got redesignated as a light bomber.
Funny enough, during Vietnam war it was given the attacker A back again.