This is the most relevant quote from support I could find on the matter:
We are aware of that the AI of the animals is yet not perfect. We are constantly updating this system to improve it further. If the creature gets stuck and cannot reach you (for whatever reason), it should fall into a trapped state, meaning that it cannot be damaged or interacted with. If, for some reason, the creature is stuck, but does not fall into the trapped state, this is an exploit. If you ignore this fact and you still interact with the creature to gain advantages you are abusing an exploit and thereby violating the EULA.
Another precedent, which unfortunately I cannot locate at the moment, is a statement from a dev explaining that keeping extra mobs (that you're not currently engaging in battle) away from you using terrain (I believe pillars in CP) is NOT an exploit, but a creative use of the environment. The EULA and TOS themselves don't offer much else in the way of specifics. More examples would be helpful, but based on these, I would make the following judgment:
If I understand the situation correctly, this does not seem to be an exploit. If the mob is able to attack one or more of the people battling it, then it should not enter a trapped state while targeting one of them. It should enter a trapped state when it engages with a target that it cannot reach. In this particular case, the mob is attacking other players most of the time, and can reach them, so it should not be trapped most of the time. It should only be trapped while targeting the players out of reach. Nothing seems to be occurring to interfere with the "trapped" mechanic's proper functionality. This seems more comparable to using terrain to avoid multiple mob agro, or using terrain to escape from a mob on a TP run, or ping-ponging a mob back and forth between two hunters to survive more attacks.
With that said, all colonists are required to obey the instructions of MindArk and planet partner devs. If the one at this event was telling players to move, then they must do so. The scope of such an ad-hoc instruction, and its usefulness in evaluating scenarios after the event, would be up for debate.