This is true of all peerless. I've actually noted changed in skill of .1, no more, and resists by 1. Disco is completely worthless for what it is meant to do on most bosses. It is a known bug that has been ignored by the devs for at least the past 7 years that I've been back in the game.
However, on Dark Fathers, and I believe Shadowguard bosses (I will check this week some time to verify if I can) it works as intended. I have not done ANYTHING boss related since pub 95 released so I need to go check again, but I've actually dropped disco off almost all bard combo chars I have for peace or some other useful skill due to the complete lack of its usefulness. It's sad, but I think they may have realized how powerful 20% reductions were & just nerfed it into the ground.