That definition is for the French word "limousine"... I thought it was the same in English. You are sure that it is not a meaning used only for Cadillacs?
It is not useful to put limousines in a separate category if they are identical to the sedans when seen from the outside... For the Cadillac example, both could be listed in the same category, IMHO.
On the other hand, stretched limousine have to be listed separately, since they are much different from the sedan used as basis.