It has either been changed (quite possible, since in 2000 there were no mummies) or Liches have always been able to disrupt any other undead and mummies can only disrupt liches.

But trust me - after the fight I made sure disruption was perfected and when I saw that it was I asked Zulg if it was a bug and he said mummies can only disrupt liches (and I thought he said liches can only disrupt mummies but I could be remembering wrong on that because I didn't really give a FF at that point).