This is the Code of Conduct’s definition of cheating, verbatim. There is an in-game system that limits the number of players that can queue into each type of instance, and coordinating by any means, especially using a 3rd-party platform, to bypass that and get into the same match with other people where otherwise that would be a matter of blind luck, is the definition of an unfair advantage. Subsequently, having a large premade group, formed by exploiting nonetheless, curbstomping casual players is extremely degrading to the player experience, which is also defined as being punishable in the CoC.
This is not something that can be “interpreted” or “debated”, because it has already been DEFINED.