Autocracy isn’t absolute power? I’ve never seen it defined otherwise, at least from the nineteenth century onwards. What’s true is that an autocrat need not be a totalitarian, but the temptation of power still requires a check.
Constitutional monarchies, where the parliament holds a check on the monarch, are not autocracies. For George VI to have been an autocrat, the parliament that Winston Churchill presided over would have had to have been a toy parliament that passed (rubber-stamped) royal decrees disguised as acts of parliament.
It wasn’t just Buchanan who made the distinction between autocracy and totalitarianism. Jeanne Kirkpatrick wrote that famous essay which is generally well regarded throughout the foreign policy community.