No. Methadone was not made to combat addiction, it is just used as such because it is convenient due to it's long duration of action and rather blunted euphoria (I know someone will disagree w/me and it's all subjective, but most people don't find methadone very euphoric compared to (dia)morphine/oxycodone/what have you). It was invented because the Germans during World War II lacked a steady supply of opium poppies. But I agree with you that who invented it is beside the point.
Methadone withdrawal is worse than heroin or morphine withdrawal, and lasts much longer. The peak withdrawal affects, per your study, of heroin/morphine may be worse, but for methadone withdrawal, we're talking weeks, just ask anyone who's experienced it. Also, addiction to methadone seems to come on quick, like in a couple of days ... thus, the perception of "more addictive." Addictiveness is difficult to quantify, but as far as I am concerned, methadone is seriously bad news unless you either intend to be on it for life or are committed and capable to tapering, which is by no means easy.