Well it's clumsy, no doubt, but my contention is that it stops Zaha from scoring because he gets the ball as well as the man. The challenge 'that he wasn't really able to win' results in him winning the ball.
More to the point, whether it's given elsewhere on the pitch or not, it wasn't given here. The standard of proof you need to reach, therefore, is that no foul is clearly and obviously the wrong decision.
Even in your interpretation of events, I don't see how we've reached that threshold.
And for what it's worth, if it had been given on the pitch I'm not sure the evidence is good enough to conclusively overturn it either.