So your criteria of a musician is to play instruments?
Playing instruments is a bit overrated by those who say you need play instrument to be considered a musician. Instruments are just a way (and just one way of several ways) to express music. Michael was a better musician than many, many instrumentalists on this world....
He was a natural, music was inside of him. He played some instruments, but not on a very high level. But that doesn't really matter. I don't think he didn't play instruments because he couldn't learn how to play. He probably just didn't care enough. But anybody with a little musical talent can learn to play instruments. It's just a matter of practice, really. But whether you are willing to invest those long hours into practicing an instrument or not - to me that's not what will make a difference between a musician and non-musician. Talent is the key, not long hours of practice, that anybody can do.