I don’t believe I said anything about probable cause for searches. The rules about that have to do with what can be seen from outside the car. But the border patrol has many resources with which to generate a reason to send someone to secondary inspection, such as sniffer dogs.
Your citation does not address what is required of persons going through a checkpoint. You are required to answer questions about your citizenship, and I think also about where you are going and where you are traveling from.
By “like” I did not mean that the laws are identical to those at border crossings. I meant to suggest that they were different from what we ordinarily understand as our rights. I am certain of this, and am sorry for my failure to communicate this more effectively.