The three they are agreeing to allow through(since that would undermine their agreement). They can hardly call a sitting Supreme Court Justice extreme, so Scalia could be Chief Justice. Of course, that does not stop them from trying to filibuster anyway. But any Republican who would vote for a Bush appointee would hardly feel that selection would be extreme, therefore logically they could vote to change the rules according to this agreement. That also does not stop McCain from being an idiot.
They've called them all extreme, including Scalia.