It's weird though, sometimes he looks invincible even when the ball is swinging/moving of the pitch wildly.(Ref: his 100 in the Innings Steyn took a 7-fer in Nagpur)
Reckon It's a combination of both. But his main issue is not even technical, It's temperamental, It's not that he completely falls apart every-time a short ball/ball pitching outside off and moving away is bowled, It's that he tries to hit it for four/six all the time.
The latter kind of ball is even more of a weakness than the short ball. Anything significantly outside off is easy pickings for him in pitches without much movement but he carries on in the same way in pitches with movement in them and ends up edging to slip, Ain't no harm in leaving the ball sometimes ffs.
The short ball weakness can be more easily overcome. Despite him not having a very good defensive stroke against it, he has shown to be capable of leaving it at times, like in the last Innings in SA, It's just that he has do it more often. Trying to cut well-directed short balls for six over the keeper thrice an over = disaster.
Might not exactly be relevant for England as the pitches will likely be much better for batting than in SA but sometimes it's more hurtful to the team if he scores 30(23) and ****s off than if he lasts out the new ball with 25(72). He's not Afridi or anything, He previously has shown the ability to last at the crease without having to try to slaughter every ball.
Hoping he has a good series and tears that **** Swann apart.