I think pretty much everyone in kpop is an artist, but I am very aware lots of people don't. Why?
As for me, a musician is someone who uses an instrument to create music. They do not have to write, and I think its weird people apply these standards. You would call a flute player a musician, even if they didn't write their piece. Idols sing, they are musicians. Anyone who dismisses Chen, someone who learned presumably for years his craft, as not a real musician, is absolutely wrong.
And for artist, it is someone who practices various creative arts, and kpop idols have 2, music and dance. Its so weird to say that if a kpop idol did not write a song they are not part of the process, as if recording and adding your own flavor to a song, is not part of getting the song to your ears.