You're forgetting about the Constitution-Free Zone within 100 miles of all points of entry including international airports that covers essentially all of the 48.

This is a misunderstanding. That's the area in which the border patrol has jurisdiction to can conduct very limited searches of vehicles and operate checkpoints without individualized suspicion in order to enforce immigration law. It does not allow searches of electronic devices.

There is a separate border search exception at the point a person actually enters the country which does allow searches of electronic devices. US citizens entering the country may refuse to provide access without consequences beyond seizure of the device; non-citizens could face adverse immigration actions.

To be clear, I do think all detentions and searches without individualized suspicion should be considered violations of the 4th amendment, but the phrase "constitution-free zone" is so broad as to be misleading.

I am not. You can still assert your rights at border points. It is very inconvenient. I have done it. If you are returning from international travel there is little they can do. If you are trying to leave the country they can make that difficult to impossible. Otherwise your rights still apply.