no, read closer. it is an exploit, possibly bannable, to shadowstep someone who is on a wall, parapet, other side of a gate, what have you, while the gates for that castle are up. that is: using an ability to gain egress to a castle while it's gates are up.
you ss someone on top of the castle to kill them because apparently kill count is your top concern, you hop back off the castle. you do it with the intent to get you inside to wreak havoc, and follow through with that intent to wreak havoc, you can be frapsed and banned. you do it on accident, and choose to say 'hell with it, i'm in, ima kill buildings', you can be frapsed and banned.
it's not saying shadowstep is against the rules. it's saying that getting into the castle in any way short of a gate being destroyed and you walking through the void where the gate was, you are now a target for frapsing and banning.
it's a pretty clear cut thing, it doesn't matter what your ability is. if wardens had a pet ability that let you teleport to the pets location, and you sent your pet inside, through a gate that blocks players, to attack a factory or a library, or anything you could target, would it be ok to use that? no. it's getting you inside the castle through some way that is not 'walking through a gate that is currently down.'
i bet you'd raise a stink if you had that and weren't allowed to use it. but dink'll take care of all that.