Im not sure they got their attention because only initials were provided in the article, but i do agree with your sentiment. Also, art is such a subjective practice. If there was a mathematics case i would be much more compelled to believe this.
Here's one related to savant syndrome: "Daniel Tammet first came to worldwide attention in March, 2004 on international Pi Day (3/14 of course) when he recited, from memory, Pi to 22,514 decimal places."
However, Tammet has been a savant from childhood, so he's a case of a better-known phenomenon. He didn't suddenly acquire previously unknown unusual abilities in adulthood. (Learning conversational Icelandic in a week is amazing, but he already anticipated that he could do it!)
Actually, he had a different name and was a memory competition champion who practiced every day. Instead of mentioning that, he decided to blame his entire abilities on a fever he had as a kid. Not to put him down or anything, but a lot of it is PR.
Under the name Daniel Corney he even advertised PSYCHIC services. Definitely his abilities are incredible, but he also spent hours and years practicing them.
Oliver Sacks had some case studies listed in his book "Musicophilia." One was a non-musician doctor who, after an injury, developed an obsession with Chopin's Fantasie Impromptu. He apparently bought a Steinway and learned the piece in a year with no instruction.
From the article, it doesn't seem that unusual: Dr. Cicoria didn't suddenly become an artist, it still took him a bit to get there. Specifically, the "no previous training" from the title simply doesn't apply.
The sudden part was gaining interest and passion in music - and it can be linked to him being struck by lightning.
As for the musical process - hearing music in his head, desire to listen and play, hearing his own music when starting to play other pieces - that's not at all unique. I can't vouch for everyone, but as a musicmaker, I can say that this is how a lot of people make music.
I think it's not that much unlike the flow state in coding, where one wakes up with the problem already loaded in mind, has ideas about structures and algorithms in the shower, and then figures out how to code it all during the day. Writing down the notes is - literally - coding music.
I think that if you can get a song stuck in your head, you also have this potential to hear new music in your head, and thus to make it. It just can get easily drown in all the other noise that life generates. Some people just need a shake to turn the other noise down.
Are there any provable cases?