And what if this person had opted to instead ship their phone - unwiped - prior to boarding the plane? He then chose to carry a different "travelling" phone with barely any personal data on it when journeying back to the U.S.

He could have theoretically done so, yes. If he had, then there would be evidence available to counter the government's claim that the evidence proved that he wiped the phone using the "duress" feature of the OS. That's another great feature of the criminal justice system: in addition to the presumption of innocence, the accused is allowed to submit evidence in their own defense. I think the phone owner would have to provide that evidence - the scenario above is unlikely enough that a jury could (and probably would) conclude that this mere possibility doesn't create "reasonable doubt" that he wiped the phone on landing absent some proof that it actually took place.

Is the argument that no normal - non guilty - person would do this and does this then provide reasonable inference that the person must have incriminating data on the phone that they shipped, which is thereby subject to seizure in transit?

None of this involves assuming or proving that there is or was any incriminating data on the phone. This is an obstruction/interference type statute: you're not allow to destroy things in order to keep the government from lawfully searching or seizing them, independent of whether those things are or are not evidence of a crime.

Now, if you were to do this...unusual thing and mail ship your cell phone to your country of destination rather than carry it on your person like nearly everyone else, and the government found out you were doing it, I think it very well might lead them to open your shipped package and search the phone. It's pretty suspicious. However, they don't need any reasonable suspicion to open your package and search your phone. Just like persons crossing the border, shipped packages and mail that are crossing the border are subject to search and seizure without a warrant or reasonable suspicion under the same border search exception. The government's allowed to examine everything that crosses the border, person or property - any and all such searches are automatically reasonable under the Fourth Amendment without any further showing.