Ideas can't be copyrighted. Design and Art can.
Main points of their lawsuit report against muntun were about this: Using garen splash, using same map, same monsters. Magic Rush was even worse, because they copied entire champions (design, art & effects). Just having 1-2 skills with similar ideas but that have clearly differences prob would be hard to argue about copyright in a court. If that could happen, valve/blizzard could also throw a lawsuit against LoL.
Although, I think truly reason is because they're from different countries. Lawsuit against muntun must be applied on China and court would prob benefit muntun over an American company. This is why took all this time for muntun being processed: Just when riot became part of tencent (which is if from China) they truly throw this lawsuit. Before, riot just gave up, because it would prob be hard to win in that circumstances.