He has seen it and that's the point.
What is the point of having someone in charge of a game who hasn't got the bottle to make a decision?
He should of yellow carded him at least and it was done.
The one simple question City may be asking is "using the same footage to find Aguero guilty,please explain how the ref has not seen it and we will accept the ban".
As has been mentioned, the term "secondarily to a challenge" is fairly vague. It means that refs can say that they were focusing on the challenge, didn't notice the violent conduct even though they were looking at the challenge. It may be disingenuous, but it makes it very difficult to challenge the ref's integrity. Which is probably the intention.