I've spent quite a lot of time in graduate art training and I'd go further: this isn't about a high level of technical skill or performance, it's about personality. A composer is not a pianist writ large; a software engineer might be a coder working at a high level of abstraction. But an artist is someone who works with their whole personality, engaging with the craft of painting (in this case). To have a "whole personality" that is compelling is very rare, and it's not something that happens by stacking up skills. It might manifest through an appearance of highly analytic vision or graphic composition but it is not quantifiable in those terms. It has to do with wit.

I'm going to cheekily (not in a sexist way) link this to something said by the fashion photographer who discovered Melania Trump, because I think it is exactly the same ineffable quality that is under discussion:

'Jerko thought at first that Melania had a very good future in modeling but after two sessions, he lost enthusiasm. “Her exterior was very good to be an excellent model, but she lacked energy, a certain charm that if you have, you transmit it through your eyes, through your personality. If you have something that, shall we say, comes from the heart, it shows in the photo,” the photographer said.'

Yes, exactly, charisma.