As I said they might have their own system which we do not see or its just another flag. Haven’t looked into it but ARK uses a heavily modified version of Unreal since Unreal is just not made for this kind of game.
I would recommend looking into it a bit more. Might be possible that you just can use the default AI movement controls and its working just fine even if the NavMesh is not there. You can easily do some tests after you created a very basic AI.