For a couple of things really, First, he did not punch in any sort of anger, Joyce was just going through the motions and making it look like he was punching, Second, The basic defence of that one-two, it was sent by Zhang like a carrier pigeon and Joyce was so slow to see it,
For a heavyweight boxer to defend like that is criminal, at this level it only takes one punch and you are out
Southpaw or not any boxer worth his salt trains for a good 6 to 8 weeks and works on his weaknesses, Joyce has had 2 fights with Zhang so should be working out Zhang but both times he was just cannon fodder.
Joyce's boxing career is over, simple as that