Lack of mention of lithium battery with older flash is likely because Lithium type battery for AA size did not exist or was not in common use at the time those flash units were produced. It makes sense that manufacturer does not want users to operate flash with anything but power source which the circuit was originally designed to handle. Too low or too high inconsistency may just be a translation error Metz being a German company.
Hold on tight. It took Metz few days to answer my question. But they did include a full and complete answer once it did answer.