There have been multiple reports of people getting to the gate in time to board the plane (door open) and being denied, because they were not present by T-15/T-30. If they've already cleared a standby into that seat, or if they've already computed final load numbers, or... etc. "Inside of scheduled departure" is definitely not the cut-off; "before we close the door" might be. But the problem is, if you assume that a passenger will show, and then they don't, and their luggage has been loaded, you have to pull that bag. That's time-consuming, so they have to give a time buffer for that.

If you're not at the gate by T-30 for an international departure, your seat is at risk. You might get lucky sometimes, but it's still a risk.