I like to think of it as a welcoming to Inkling as a character. Smash invites, to me, are considered the greatest honor for a game character, and it's even more of an honor to be chosen as the first newcomer for the series, as it was for Meta Knight/Pit/ZSS/Wario/Snake and Villager/Mega Man in the past. This was Nintendo's legends (led by Mario and Link, natch) announcing the new game, and telling Inkling that they have been given the great honor not just to be a Smash newcomer, but also that they have made it to Nintendo's pantheon.
Besides, everyone knew that if there was a new Smash game, that Inkling was an absolute given for being a newcomer. This was Sakurai just making it official.