Maybe some who has been through this can tell me the real story ....

I am ticketed on AC 5160 YYC/DEN (AC Code share on UA1164). I called Diners customer service to see if I would have access to the MLL, the answer was "Yes".

Then on a subsequent call, I posed the question again (thinking that the first "yes" was a bit too easy) and added that the flight was being operated by UA. With that knowledge, the answer was then "no".

Is that correct? In order to have access to the MLL, I need a AC boarding pass on a flight that is operated by AC? Or can it be an AC code share on UA metal?