The wording even makes complete sense with Pein‘s claim.
Normally, the action is unstoppable.
Though if his target is hiding, the target becomes immune against Strongman kills instead, not unstoppable. And Pein‘s target is blocked, so that adds up, because making a blocked target‘s action unstoppable would go against Pein‘s ability, so this way those abilities don‘t mess with each other.
So unless there‘s something else to that, this should confirm him.