It seems clear they didn't send out that many copies, Nintendo has done this before with games like Luigi's Mansion 2 and Fire Emblem Awakening. The store I went to yesterday just a few hours after it opened only had 3 copies left. I doubt there was a huge rush as soon as it opened, so that most likely means they didn't have many to begin with. However it is Kirby, Nintendo should have a pretty good idea of how it sells, so still a sign it's performing better than they expected.
As for the game itself? Unsurprisingly, Kirby continues to be the most consistently good series ever!