They wouldn't need to be heavily in the music sphere. Just a "concerned individual with insider knowledge." You know, someone from her vtuber company contacts someone that she's working with and says, "Hey, I don't really want to make a big deal out of this, but I figured since you're working with Lumi, you should know that she's X, Y, and Z. Please don't mention my name, because I don't want this getting back to me." Then they spread the "insider info" to their friends, and it spreads to their friends' friends, and so on.