I think this is where your angst has merit. The GA lacks authority to close the flight before T-15 without all checked passengers on board. To threaten you with flight closure is certainly power tripping and abuse of authority. They can sure beckon you to board so they can close the flight, but they should not threaten to close the flight without you, other than saying the flight will close at T- 15. Any of us can be in a situation where we arrive at a gate T-15 to T-20 and to see a closed gate is unacceptable, other than an IDB in a full flight.

That said I find it a highly risky proposition to deliberately arrive a gate at T-18 - do that enough times and something will happen that will prevent you reaching the gate by T-15...