On the surface, it looks like just one person is effectively working on this: Mell.P. Their twitter is just a normal person's twitter with these music projects featuring
Mell sprinkled in.
There are no posts that say "I'm releasing a CeVIO AI VB!" or "this is my current project!" It seems like they just started doing it.
They stealth launched the utattemita career of the Mell character. When met with confusion about what their announcement post about their activities meant (it was tagged with vocalo and vtuber), they said that they're a bunny between vocalo and vtuber,
And in other posts, it says that they were "born to sing".
I am assuming Mell.P is Mell and this is their personal project.
There are zero videos on niconico about Mell (which would be the first place I would go if I were trying to market a new synth vocal and go straight to the users quickly), nor any posts officially announcing a CeVIO AI bank.
This user pointed out that there's nothing on SPYNews about Mell, and yeah, I agree- it's kind of weird! It totally stealth launched.
None of the videos or posts have much interaction, so honestly, yeah, if I was just a passerby, I wouldn't think much of this, probably. If you're paying attention it's obvious that Mell's singing in the videos you linked is a synth, but if you're passing by, you'd probably assume that it's some processed vocals for a vtuber project and not necessarily a synth. Nobody knows who Mell is, so if the goal was to get a lot of people to buy Mell, the titles of the videos are probably a mistake.
I think this is an individual that happens to have the resources to do something like this taking the plunge without any knowledge of marketing.
It kind of makes me like the project more if this is just some person making something that they like.
(EDIT: of course, I'm assuming everything going on here is above-board. I'm not seeing any major alarm bells other than the silent nature of the project, and maybe the fact that techno-speech hasn't tweeted anything about it. But my gut is saying it's real, just not professionally handled.)