But isn't the thing about imposter syndrome that it's not about whether YOU think the musician is good, but whether THEY think they are. As an amateur musician I can totally understand feeling like I'm really not that good, even if other people tell me I am - I just assume they didn't hear that sour note in the 3rd lick I played, or that they didn't notice I should've played a certain phrase more staccato. Especially when I hear recordings of myself, all I hear is the bum notes or the missed opportunities to do something better than I actually did. Even if I put on a performance that people say they enjoy, I often feel like it was just dreadful and I shouldn't have been on stage in the first place. To me, that's exactly what imposter syndrome describes.
(Not trying to throw a pity party...I love music and will happily keep playing anywhere that will let me for the rest of my life.)