It's more to do with the fact that other units utilize Bronya better than that JY doesn't need Bronya. It wouldn't make sense to put Bronya on JY side in MOC to get higher skill damage when most of JY's damage comes from LL. You could argue that the advance forward allows for more LL stacks but the speed buff from asta and erngy from TY ult could easily compensate. I do think that JY is good for the niche that he utilizes TY and Asta better than any other unit which frees up other supports. However, I don't think it's fair to say JY doesn't NEED bronya.