As good as the theory may sound, I don't think United employees are that united in attempting to screw you over. I can see GA deceiving you to lower their VDB/IDB count, but front counter has nothing to gain by doing this. At a small airport where it's the same person at front counter and also at the gate (they move around) I can see them doing this also, but at ORD, no.

Basically rules are rules, and the front counter agent has no incentive to bend the rules to let you through or work extra fast or efficiently. They do stand to suffer if discovered by United to not follow procedure.

On the departure monitor, did it say that 8pm flight was canceled? If not, the front counter agent didn't know any more than you do.