A croll can be seen as a point of attachment but it may never be shock loaded. In this climb it doesn't matter though because the cows tail is also attached to his D-ring. Use in aid-climbing is allowed but not recommended because it can make rescue a bit difficult, better to just put an alpine butterfly in the cows tail to shorten it or use something like a Progress adjust or even better: a Grillon. At least from what I've heard at my level 1 training last january and seen in some explanation video's of L3's who are explaning an aid-climb rescue.