Published by Nintendo in the US.
Chart is about 3rd party games, doesn't matter if it's published by Ninty or not. It certainly not first party, nor is it 2nd party. It is on PC now too.
zorg is right because that's how NPD does these things. Nintendo published Octopath Traveler, so it counts towards their total of games sold in the US, plus it excludes the game from a third party game list because Nintendo is not a third party publisher.
It's true that Octopath Traveler isn't a first party game, just like it's true that Mario + Rabbids isn't really a third party game. There are notable differences though. Octopath Traveler is merely a third party game that was published by Nintendo while Mario + Rabbids is an IP collaboration which puts it in the same inbetween area as what is defined as a second party game (console manufacturer working with an independent third party where the console manufacturer has control over which platforms the game arrives on due to IP and/or copyright ownership).