Should this leak prove true, it definitely changes things. For one, for a variety of reasons, I'm pretty sure Skull Kid will end up being a boss. I doubt Sakurai would leave him out of the game entirely after all the acclaim he's had, and if Sakurai knew Breath of the Wild was on the horizon, he may have just decided to deal with potential Zelda representation for whenever that game had come out, and go with whatever fresh new character had the most support at the time. Also, if you'll notice in the Youtube video where the moon assist trophy is first revealed, the clip opens with a glimmer of sparks on the left side of the screen that fizzle out just as the moon is summoned, which look to be about the size of Skull Kid. While the sparks didn't appear close enough to be ascribed as an attack by one of the players judging by their positions, it could easily have been an assist disappearing from view. If Skull Kid is a boss, my guess is that Skull Kid's small stature would play to his advantage in a fight, with your character attempting to inflict enough damage on his elusive self before a moon (approaching the screen at a slower pace) destroys you. Pure speculation here, but it would also align with LeakyPandy's tips, and seems like something Sakurai would do. Sakurai's most creative efforts have not been directed at Zelda very often, with half of its entire cast borrowing movesets from other characters, suggesting it is not one of his favored franchises. Skull Kid is a character that would take some work to bring to life, and it looks to me like most of Sakurai's creative energy went in other directions, like Mach Rider or Ridley, especially given the tight schedule his crew was working within. While its clear Sakurai had Skull Kid on his radar, his poor performance in polls likely influenced his promotion to boss assist trophy rather than full-fledged fighter.
Then again, Sakurai may have also left the character out entirely in anticipation that fans may want to give him a second chance for playable status when he'd have more time to work on him. He would certainly bring in the moneybags as DLC! I want to clarify that SK is my most wanted character for Smash right now (I mean, check out my avatar!), and I plan to participate in any campaign to make him playable (maybe even if he gets boss status).