This is only true for United *Gold members, but as AC Elite you are entitled to access, even on domestic itineraries. I have never been denied access, but it does occur occasionally.

The UA agent was mistaken, and there is likely verbiage on United's web site to back this up. I would definitely report this. Hopefully you got the agent's name.

Ironic, since United Clubs are likely the worst lounge offerings across *Alliance.