Target command only works if there is a target; the target is not shared, meaning only the player or Helper that actually hit the opponent has a target. What you want is a double redirect like root,target,command, which isn't possible in that exact form, but can be replicated with the cond() exploit.
For this particular example, it would look like this:
type = changeanim
trigger1 = anim = 4000
trigger1 = root,cond(numtarget,target,cond(1,command = "holdback",0),0)
value = 4007