Unless I'm misreading this is all covered explicitly in the article.

The article claims multiple times that the cops used the interstate travel as a pretext for the search that turned up the marijuana, but in most jurisdictions, the arrest would have allowed them to inventory the car anyway as part of the impound process.

Unless this jurisdiction is an outlier, the pretext wasn't needed for the search. As the parent comment says, the cops used the interstate travel as a timing strategy to increase the charges.