I think you just have to be a bit more flexible in understanding what was intended by "double the amount of information in one character". In fact, as you point out, the shortest translation of あほ would be "fool", which has twice as many characters in English. That's the sense that's intended.
The CJK languages don't use spaces and many words are only 1 or 2 characters long.
Not quite 2:1, but you can see the point.