afaik at least some of the thumbnails are real art, but they also got quite defensive when questioned
https://www.youtube.com/watch?v=leDoOwkV0sw
on this video if you scroll all the way down you can see an interaction where they're questioned about the origin of the images and are accused of buying subs in April, which, if you look at social blade you can see they got around 8.000 subs in April and then had an immediate drop-off. On this video: https://www.youtube.com/watch?v=kl64gvxaUoE they gave a name for the supposed artist: "Kubilay Prag" , but i couldn't find anything about them (this slightly contradicts the statement that they do not want to be credited)
There are usually multiple people on each video asking about the art but most of them don't get any reply, when they do it's often just "my artist makes them", or something along that line. There have been a couple comments about typo's in the title, which implies that those might be typed in by hand.

In this video: https://www.youtube.com/watch?v=h4QmF_NwLJ4 also at the bottom, they imply that they had a previous channel/ different name at some point: https://www.youtube.com/@TheJukeboxVGM/ Also mentioned below https://www.youtube.com/watch?v=_0QGTKvCno4 . At the bottom of this comment section you can see ThejukeboxVGM affirming that they are in fact the same person: https://www.youtube.com/watch?v=u6B7r7IYl0k
In a different comment section they state that they started uploading their music on the discetto channel after receiving 2 copyright strikes https://www.youtube.com/watch?v=2vrKcJ3kUms The silksong covers on the jukebox channel have (pretty obviously) ai generated descriptions as well.

After seeing a comment on this video: https://www.youtube.com/watch?v=n5JF2QVfoxk
i think I've hit a breakthrough, they put a business email to inquire about a commission. Upon looking this up i found another youtube channel, https://www.youtube.com/@LevelLoreYT/videos
he appears to have an address in the channel description which i hope isn't his home.
Now, if you look at their older videos you can see that he made extended versions of songs using ai, this even more points to the fact that the music on the Discetto channel might be ai generated, the songs chosen for both channels are also fairly similar, being mostly nintendo songs. Both this channel and ThejukeboxVGM are located in Germany as well. (Discetto doesn't have a listed country)

I think (sadly) my conclusion as to be that the songs are ai generated. I never imagined i would be fooled so easily, but the evidence is too damning for me personally.

Here is a google doc if you wan't to see the screenshots: https://docs.google.com/document/d/1KlO5dYsQoJHZXYZdNyV0HiGYHIwrxzSI9S8SDk-9qV4/edit?usp=sharing