Actually, I think he could still be using existingmummifiedwell-preserved bodies as a modification to the regular spell. Unlike animate dead, which has been used quite often, create undead's spell description states it has a one hour casting time and can only be cast at night. Six mummies would require six hours, usually. It was day before they entered the pyramid and we can clearly see light outside the window in panel four. As far as I know, a scroll uses the original casting time if it is longer than a standard action.
Malack could conceivably have a magic item which casts the spell and allows for a shorter casting time. It wouldn't be his staff, though, as that kind of magic item also requires the spell's casting time if longer than a standard action.
Then again, when Tsukiko made a ghoul, it was (probably) a standard action.