Border control agents are allowed to search his phone without a warrant because he technically wasn't in the US, even though he was in Atlanta, because technically that part of the airport counts as not being inside the US.
But, the border control agents are allowed to search his phone in a part of the world that isn't in the US because...? Are border control agents allowed to search his phone in France or Australia? Or is it only in these special pocket dimensions that occur in airports that are in the US but not in the US?
When they demanded that he unlock the phone, he wasn't read his rights, and they refused his requests for a lawyer, presumably because he didn't have those rights because he wasn't technically in the US at the time.
So, he gave them the "duress code" for Graphene OS which deleted the phone's contents.
Now he's being charged under a federal law for knowingly damaging or destroying property to prevent it from being seized.
Is this a law that applies outside the US? Because, we've established that what happened wasn't in the US, which is how they were allowed to search his phone without a warrant, not tell him his rights, and refuse his request for a lawyer.
I know if fundamentally comes down to "might makes right", but borders are an especially egregious case of a location where your rights don't matter.