I was booked today on a United flight XXX to DEN

I was unexpectedly able to get to the airport earlier, so I decided to standby for an earlier departure.

I hurried to the airport and arrived at the gate 25 minutes prior to scheduled departure. At that time, the gate agent brusquely informed me that no one could be added to the standby list after T-30 minutes.

I was disappointed with the agent’s rudeness and with the lack of effort, especially since the flight was fully boarded and had empty seats available well before scheduled departure.

I have read standby terms and conditions at https://www.united.com/ual/en/us/fly...t-changes.html and found nothing referencing a 30 minute cutoff time.

Does this policy actually exists? If so, where is it documented?