as far as I understand, they were not there to serve a warrant. he was crossing a border and the feds are claiming that the 4th amendment doesn't apply because he hadn't yet crossed into America.
border patrol was essentially using a loophole to preemptively collect potential evidence (they'll figure out what to charge him with after they see his phone contents). he's a political activist against a cop city, they were obviously trying to do him dirty.
they were not serving a search warrant. they were abusing what should be an illegal search at the border in order to silence those who speak out against govt overreach.