While the way it's applied may seem like it doesn't make sense, it's really a balance thing from a design perspective. TF's W was made to be a little less like standard attack modifiers because he depends on it so much. So when he uses it he gains additional attack range for that particular use, so it's more like an ability that he auto attacks with rather than an attack modifier. It's a subtle difference but it's to help make sure it has additional reliability compared to other attack modifiers, it cannot be blinded, for example, or as you noticed against Pantheon, it cannot be blocked by his passive, though to be fair the damage portion of the auto attack (his base ad for like 80 damage or so) is mitigated by blind and blocking effects.