Short Version: you need to replace line 57 in CanSeeTarget2d.cs with this line, because the gizmo is pointing the view angle cone in the wrong direction: Gizmos.matrix = Matrix4x4.TRS((Vector2)agent.position + offset, Quaternion.LookRotation(agent.right), Vector3.one);
Longer Version:
Before changing the code the View Angle cone shoots out in the z axis, and is not representative of the actual cone. You can observe this when you view in 3d.
The actual trigger is in the wrong area marked in file #2.
The pasted code (thanks to @aFoolsDuty) corrects this.