Why was Bumrah's boundary-line effort called a 6 in SA T20I?

During South Africa's seventh over in the first T20I, Jasprit Bumrah jumped and threw the ball inside the fence while airborne, but umpires signalled it as six. It was declared a maximum as Bumrah's feet were touching the rope before he jumped to catch the ball. If Bumrah would've jumped from inside the boundary, it wouldn't have been a six.