I haven't read the latest arguments but is the jist of it that City are claiming the league is rigged because 'Big' clubs with huge global commercial revenues (i.e. liverpool, City) have an unfair advantage over those clubs without such revenues.
Sounds a valid argument to me. Why can united spend freely and pay players 400k a week, but everyone else isn't allowed unless they can sell 1 million Ronaldo shirts in China.
However, that doesn't mean you are allowed to break the rules as they are written.