So the physical state P_1 leads to an already predetermined physical state P_2 i.e. the player lacks agency. When you say to pick OB, what you are actually saying is that you want the physical state P_1 to reflect an agent who will OB.
The difference is that when I say pick OB/TB I am not trying to change the physical state P_1 because you can’t change it once you are in the room. In your view, once you are in the room, the agent simply follows what their past physical state leads them to do. So no choice/agency/freedom however you want to call it, there is no decision once you are in the room.
You need to understand that nobody even two-boxers is saying that prior to the prediction you should appear as an agent who TB. So if you interpret “what should you do in front of boxes” as “what should be your physical state before the prediction” then yes, you should OB even CDT says you should, but this isn’t the question two-boxers are answering.
We consider that the physical state in the past doesn’t predetermine the choice in the room otherwise obviously there is no choice in the room and you are forced to interpret the question as a completely different question.