The guy from the article was referred to as "a US man," so I'm assuming he has US citizenship.

The border cannot legally deny US citizens from entering the US. But they can do all sorts of other things, like detain or delay or prosecute people for not complying.

He's a US Citizen. Legally they cannot deny him entry into the country.

Central to these protections is the absolute right of a U.S. citizen to return to their country. This right is unequivocal. No matter where an individual has traveled, the duration of their absence, or their personal or legal background, a U.S. citizen cannot be denied entry into the United States. This principle is supported by both statutory and constitutional law. A landmark Supreme Court case, United States v. Wong Kim Ark (1898), established that citizenship includes the right to reenter the nation. Unlike lawful permanent residents or visa holders, U.S. citizens are exempt from admissibility criteria. Consequently, even if a U.S. citizen lacks proper documentation or is subject to further questioning, the government is obligated to permit their entry once their citizenship is verified.

That's what happened

That is what happened. The accused told the boarder agent the duress pin when asked. The agent entered the pin that initiated the wipe.

Could make a difference at trial, didn't stop the case prior to the arrest.

they have the lawful right to seize and inspect your phone

Sure. They can look at the outside of the hardware all they like. But if they can't access the data on it, that's (Constitutionally-speaking) purely their problem, not ours.