I once worked with a singer who had a good voice but would argue that you played in the wrong key. NEVER do that with a musician ! He'd often come in on the wrong pitch and it made ME look the bad one. He built up some bookings riding along on the back of his famous brother. He was the type of person who didn't pay up fees on time so I told him where to shove his work.
I'm the leader, I get the biggest share, the box player's next he gets next. The drummer gets the smallest. Totally selfish & wrong.
I remember one of the best I ever played to was a Japanese woman.
I announced a bveautiful tune called song of the seashore. She asked if she could sing it. I agreed. WOW ! What a magnificent voice, almost angelic.
My view then ? yes a singer IS a musician if he / she knows how to use their voice properly. Many I've worked with knew what keys they sang every song in and were happy moving up from D to Eb for the last verse of a song. Very effective in Scottish music.

Graham............

https://www.youtube.com/channel/UCKBPq93IQFTD3q-zBqxghbQ?view_as=subscriber