You misunderstand, Mut. Not broken as in "OP broken", but broken as in "it doesn't work as written".
It replaces the extra deck with Goyo Guardian instead of adding it.
It would be considered "Bugged" if it is a bug. But I heard people said that the actual yapanese description says, that your extra deck BECOMES Gojo Guardian. So if thats true it is only a translation mistake.
I hope not tho because this would make the skill extremely poor
Unfortunately, yes, that is how it's supposed to work. (They mentioned as such on the "Issues" tab in-game. It's only the description that is incorrect.)
Anoymous
Mut