I am not so sure Archie. The ball was shown to be hitting bang on the centre of off stump, not just edging past it. Secondly the ball had pitched outside the off stumop and moved further in towards the batsman's pads hitting it in line.
The umpire tends to , subconsciously 'track' the ball, from the time it first appears in his vision, very much like the batsman and the keeper do. This does give him an idea of where the ball is heading. It also, sometimes, leads to faulty decisions, if the ball does something completely unexpected at the very last minute. If he had been tracking that ball and had noticed that movement (against the normal turn of the off break) making it a doosra (or at least a straighter one going in with the lie of delivery, eh would have formed a mental image of it hitting the off stump.
I hope you get my point.
Suppose it had been an off break instead and had landed on the off stump (instead of outside as it actually did) and then turned just enough to straighten to go on towards the off stump, the umpire would be in a bigger doubt since he may have noticed the away movement. In this case he was more likely to give the batsman the benefit although the tracker may have shown even that delivery hitting in exactly the same spot as this one actually was.
I hope I am clearer than I think I am