This came up in (as of this post) the most recent BoLS blog post, and frankly I hate blog comment conversations so I'm moving it here instead.
For my part, I'm consistently annoyed by people claiming someone is breaking the rules than trying to backtrack saying they're not saying the person is cheating. That's the very definition of cheating in gaming-- violating the rules of the game. If you were to move your king as if he were a knight in chess, that would be cheating because you are breaking the rules of the game. If you were to claim every lasgun in a horde Guard army was a lascannon, that would be cheating because that breaks the rules (C:IG defines a lascannon as S3 AP-, not S9 AP2). If you were to use an aimbot in TF2, that's cheating on most servers as most servers have rules against it, and therefor it breaks them.
If a player is playing by RAW, they are not breaking the rules no matter what army they're using...