It literally isn't. Nintendo comissioned and published it, they paid for the game to happen.It literally is. Long dormant, much beloved franchise, used as branding and thematic window dressing for a game in a different genre by a different developer. The way it sets up and then crushes expectations in the space of its title reveal is very similar.
In this situation, a indie dev asks Valve if they can do a crossover and Valve allows it, just like it has allowed it countless times in the past.