You may choose to blame AC, but the UA agent has to know how to do this and not refer you to AC. This has been discussed over the years ad-nauseum: No matter how much goodwill, effort and seeming brilliance or kindnesses by AC agents, you will not be able to confirm a seat on AC metal when booked as a UA flight number UNLESS you find a UA agent who knows how to generically request (aisle or window, for example) a seat for "their" flight via "their" PNR; and even then the response is not instantaneous. If you are told otherwise, you may feel placated and happy, but be prepared for possible disappointment when you checkin (if it truly matters to you for such a short flight).