You're correct that 2nd party doesn't really exist in the industry, but what actually solidifies if a game is first party or not isn't publishing rights or who developed the game, it's IP control.
Nintendo confirmed this when they listed Astral Chain explicitly as a first party IP even before owning the entire IP. If you own a controlling share (which Nintendo do because they own parts of the other companies invested in Pokemon) or half or more of an IP, it's first party to Nintendo. So yes, in this scenario, Just Dance 3 is the best selling third party.
Pokemon is owned by the Pokemon company. While Nintendo does own a piece of it, it's 32%, not a controlling share.
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