That's not just about the game between Manchester United and Manchester City, in most of the games Macnshter City had, against strong teams like Bayern Munich and Real Madrid they had amazing performances they had amazing results and this a good reason to say they will win champions league title too.
You are very correct, Manchester City have played against most of the big teams in Premier league and champions league, and they have performed very well . Manchester City playing against this top teams, they never struggled. The record of Manchester City playing with this big teams tells they are good to defeat any team.