What behavior are you seeing exactly? I just tried this exact thing- FollowerEntity with a FleePath being set as their AIDestinationSetter destination with no issue.
[Button]
public void SetFleePath(GameObject fleeFrom){
var fleePath = FleePath.Construct(transform.position, fleeFrom.transform.position, 10000);
// follower.SetPath(fleePath);
GameObject newObj = new GameObject();
newObj.transform.position = fleePath.endPoint;
GetComponent<AIDestinationSetter>().target = newObj.transform;
}