I still think that the Chorus Kids are happening this game. Yes, it's true that we don't have complete evidence to validate Gematsu. Regardless, it's weird to see that some people believe it was all random guesses. The Rhythm Heaven icon was in the characters section, there was an abundance of Rhythm Heaven trophies, and the Sneaky Spirit (alongside the Polar Bear) was the only enemy in Smash Run from a franchise without a playable character. Given all of this, I think it's hard to say that the Chorus Kids weren't planned. There are probably two reasons they were cut: either they were one model walking together but had low priority, or a trio character like Ice Climbers +1 that couldn't work on the 3DS. In my opinion, the latter is more likely.
I've said this before, but Sakurai probably told the Rhythm Heaven developers of his intention to add one of their characters in Smash. I feel that any developer - especially smaller ones like them - would be very excited. For Sakurai to say this, then not add one, then exclude Rhythm Heaven in a brand new game would be pretty disrespectful. We had the Forbidden 7 in Brawl and Bowser/Dedede/Mewtwo in 64, but they already had reps from their respective franchises in the game. In this case, Rhythm Heaven still has nothing. I just have the feeling that as a fellow developer, Sakurai understands this.