"musicianship" its really a pretty vague word. i think it could be said that there are many ways to excel at music... songwriting is one, virtuosity is one, but even within those, you have people whose main songwriting skill is lyrics, melody, rhythm, chord structure.. virtuosity could be that some is an amazing soloist, improviser, or good at reading music and playing it perfectly...

really there's a complicated relationship between all the aspects of what it means to be a musician, and while you could say someone is hiding a lack of technical expertise and experience with an instrument by writing good songs, you could just as easily point out artists who hide their lack of songwriting skill by playing amazing solos, improving other peoples songs, or relying on playing fast or impressive live performance.

really they arent hiding so much as seeing what theyre best at, and going with it.