I don't see how that definition of genius shows that being a genius isn't a matter of opinion.
Whether certain people are geniuses or not will always be debated. Quincy Jones recently said that The Beatles were "the worst musicians in the world". That's his opinion. And I can't really provide any real proof that he is objectively wrong, other than pointing out Beatles' record sales, which are again based on people's opinions of their music.
I kind of get where you're coming from saying an artist can't be a genius, but at the same time I disagree.
I agree on a root level. That root level being that a craft is objective and art is subjective. But I also believe that there are artists that fit into the category.
To me a genius is someone who solves a problem. Just like I posted, Jimi taught himself how to play guitar in not just 1 but 3 ways. He wanted to play the guitar, but had no way to learn left handed, so he taught himself how to play anyway. He saw a problem, and created a solution. A very unorthodox solution, but it took some level of genius to do that, especially at the age of 13.
Stevie Wonder is blind, but plays the piano and the harmonica. He taught himself to play the harmonica at age 5. WHILE BLIND. AT AGE FIVE. He learned the drums at age 8, and we all know he can play the piano. He has an album named "The 12 year old genius", and for good reason. Not because his songs were bangers, but because despite him being blind, he taught himself crafts that required hand eye coordination, despite not having the physical ability to do so.
The level of mental algorithmic ability from people like that is amazing, and worthy of the title "musical genius". Music is just the category, genius is the ability to solve a problem. Someone who's a mathematical genius is a genius not because of their answer, but because of how they got to their answer. A mathematical genius can solve what would be regarded as the most difficult equations without a calculator. Someone who just "gets it" without having to study as hard as others or with minimal effort. Thats what makes him the genius. Mathmatics is just the platform, the craft. The genius is in the ability to solve a problem.
You said that sports are a craft, but music is an art. But music is created using a craft. Whether thats playing the piano, singing, guitar, violin, spoken word, etc. Those are crafts. The songs composed from those crafts is art. Art is subjective, but the craft/skill required to create it with is objective. Just like an athlete has to master the craft of boxing, a musician has to master the craft of playing an instrument. I believe there are masters in both.
So we agree 50/50 on it, because you said yourself that a craft is objective. That I agree with.