Since when is any cup game different from league or CL? they are all against better or equal teams, but you seem to meet higher level teams in the cup. You can have scouts in the cup and lose. You can have no scouts in the cup and win, but as you progress in the cup, if you dont have scouts, you are lucky to make it to the finals.
In the CL or league, you are guaranteed to meet opponents at your level, but there is nothing stopping them from loading up on scouts during the season or as the cup progresses and wiping you out in a flash.
I think almost every game can be won by management. I won 3 straight cup meetings vs higher level managers but ultimately lost because I got out managed by a higher level manager. Was it because his team was better? Was it because I chose the wrong formation to play vs his team? A combination of both? Who knows.
But to say the cup is all about skill and management is a only half correct. You forget to mention that the game engine and the level of your opponent are other important factors. The game engine ensures that improbable outcomes happen when you least expect it. Seriously...how can you have 80% possession playing an opponent with an illegal formation and NOT win??? Seriously??? How can an opponent play with strikers in defense, goalie as a striker and players all over the place out of position and still win???
All this to say that the cup is harder to win but lower level managers have to hope for a bit of luck to win. You still need to manage better than the other manager to win every game, cup, CL and league if you are a manager without scouts.