"Private" in marketing terms only. Border agents have access to it all. If you refuse to turn over all of your passcodes and passwords for the access they're after (including social media profiles), they can detain you and seize your device.
Border Patrol is a step above most law enforcement when it comes to authority. The Fourth Amendment doesn't apply in the 100 mile border zone, or at least SCOTUS ruled Congress hasn't defined what happens if it is violated by border patrol (Egbert v Boule).
