Again, what one gets on the phone does not necessarily reflect reality. Even the MileagePlus service center agents are not versed in the benefits/nuances of the program. Their primary responsibility is managing MP accounts.

"if a seat upgrade becomes available, waitlisted customers are confirmed automatically based on the Premier ® status of the traveler first, followed by fare class, Chase United MileagePlus Club cardholders, United Corporate Preferred participants, and lastly, by the date and time of the request. If the request includes multiple travelers, then priority is based on the highest Premier status on the reservation."