The rule is clear and both UA and LH enforce it. If you do not have your card physically with you, you run the risk, as OP did here, that your BP alone will not be accepted as "other valid indication". For those who recommended demanding the agent's name and all of that, it's best to look at the rules first.

Customer must present proof of Star Alliance Gold level status via a valid frequent flyer program Star Alliance Gold level card or other valid indication of Star Alliance Gold level status