There's no requirement you be on board at T-30. I don't think I've ever seen an international flight fully boarded at that point. For international flights, it's stated that "it is required that you be at the gate and ready to board at least 45 minutes before your scheduled departure time". However, since they frequently don't even start boarding until T-45 and you are not required to check-in with the gate (and as mentioned above the MCT for D-I DL flights is 40 minutes), it's not a requirement I would take seriously. Also, it's not clear it really applies to people from connecting flights as they state it "applies to all customers checking-in". Basically, it's likely there so GA's can call for international document checks (for those that were missed) and to discourage stragglers from showing up at the last minute to board the flight.