For goodness sakes, stuff like this happens all the time and it's just plain old poor training. There is nothing worth a thread or a complaint to UA. If this happened at GVA, I can assure you that the manager dealt with it after OP left, both because the front-line agent was wrong and because the agent argued with the manager in front of a customer.
The whole "Polaris" thing is a bit of silliness and leads to this, but I suppose that UA having chosen to name its cabins so that people think that they are something more special than plain old First or Business, has to spend the time to train its people to deal with the issue.
OP - I do think that if you stood anywhere for 30 minutes, you are being a chump. Either get in the regular check-in line or walk up to a counter and ask why UA is letting its premium passengers wait.