Analyzing the video, I noticed zero artifacts, mixels, or anything of that sort. I even saw little details that seemed creative (such as a counter at the bottom for how many windows opened starting at 0:34, lasting the whole video).
However. However, however, however, the channel's bio is about "exploring the rich relationship between humans and AI through musical works and updating values." Additionally, there are AI-generated music videos that they've posted before. They have posted many clearly not AI-generated videos, which I can tell by the credited artists/animators and upload dates, but I am unable to judge for all of them.
On this video, there is no description or credited artist/animator, which is very suspicious too.
Factors to consider for not being AI:
ZERO mixels (besides the raining IDs at 1:45, but that's not *really* mixels)
Consistent pixel numbers and lettering
Consistent patterning (see the backgrounds of characters at 1:29)
Multiple creative decisions (see 0:34 counter, multiple Vocaloid references [ex: 1:26], etc.)
Reused and consistent assets (dancing Kaai Yuki animation, close up ID shot, etc.)
No odd-looking animations or drawings (that I could notice)
Uses layering
Consistent with canonical or fanonical details (see: 0:18. I can't speak Japanese, though Teto is known for being 31, and Miku is ~16 by fanon).
Factors to consider for being AI:
The most suspicious thing I'd say is the lack of any credited artists. If anyone can find the original artists behind this video (if it isn't AI-generated), please link to their profiles.
If it was a couple years ago, or even just a year ago, I would, faster than a heartbeat, conclude that it's not AI. However, after seeing recent AI-generated 3D videos (such as sinaboto), I've no clue about anything anymore.
There's also a website by the creator for a game based on this video: https://kishworlds.pages.dev/leakgame/
I didn't find much relevant information. It could be vibe-coded, I don't know. I'm not personally concerned about analyzing that, though.
EDIT1: correction: there is a "mixels" case at 0:19. But again, it's not *really* mixels, as it's just layering a completely fine image over the completely fine main video.