[tt]target_id[/tt] is for the target overmap terrain, it has nothing to do with monsters.
The mission spawns two graboid monsters, only the second one (which is named) must be killed (the other is not linked to the mission).
I also found (through the jabberwock mission) that new monsters might not be put into the reality bubble if the mission is placed near the player. In my case the starting NPC gave the jabberwock mission, the monster was created on a forest square adjacent to the starting shelter. The jabberwock was not there, but did appear after debug teleporting away and back.