I must say I too was unaware not this rule. This 30 minute deadline is also not reflected on the boarding pass of several international flights I have taken so have in many occasions strolled to the gate at T-20 to avoid the crowd and standing in line to board (assuming I choose not to board early). Indeed I was on a UA flight yesterday from an international origin and the boarding ends time is 15 min before departure. Also I don't get how check in time is up to T-60 if gate closed T-30. Many international airports if you get to check in desk at T-63, it would be impossible to complete check-in, get through security and immigration and then be at the gate by T-30.

Certainly the rule exists but is likely there just to protect the airline in situations like this, where they advertently or inadvertently deny a passenger boarding, as opposed to being an operational rule. Very likely some other reason (other than being under 30 mins) exists for the denied boarding. They probably just didn't think she could make the connection and just closed the flight early.