I fail to see why the new music suddenly deconfirms Geno.
We're getting over 800 ****ing music tracks, and I guarantee you there are more than two or three Mario ones, especially since the series evidently has so much it's split into two on the sound test menu
Diversity in the music samples on the blog will give the impression that the 800 songs will be equally diverse and exciting. It sets a good first impression, it's good marketing.
One of the ways to help this impression of musical diversity is to avoid repeats in IP they're pulling the music from. So if they are wanting to market the game as well as possible, it makes sense to limit blog music to one song per franchise, which as I said before is easy to do.
It also makes sense for any unique newcomers to come with a new song. After all, even when we don't get a new stage (K Rool), we at least get a new song. This pattern can break, yes, but the reason why people believe it will hold is it's just good marketing.
Taking these together, a song from an IP seems to suggest they may not have a new character from that IP. Of course there's no guarantee, it's just an assumption Nintendo will want good marketing....
...but then again, Nintendo, as people repeatedly point out, don't take advantage of other easy marketing wins like tieing announcements/releases to related holidays. Sometimes some obvious marketing just isn't done cuz Nintendo gotta Nintendo.