A "learning disability" generally means that a person does not take in information properly. The "classic, textbook" example would be letter reversal: the teacher writes "dog" on the board, and the student sees "bog." It doesn't involve the actual processing of the information, just taking it in.
"Mental Retardation" (last I checked, now called "Intellectual Disability"
: this has to do with the actual info processing, whether it's taken in accurately or not.
You can be classified with one or the other or both.
As far as I can recall, L&H never manifested any type of LD. If real, they would be bumbling buffoons largely because they were not the sharpest knives in the draw -- but not retarded -- plus they were naive and sometimes impulsive.