They do take cracks seriously, particularly on short hop ac. Aloha and more recently SW have had metal fatigue incidents on AC that have had a very high percentage of cycles to age. I believe you are a commercial chemist or materials expert so you know far more than I on the subject but the way I understand it is the pressurize / depressurize cycles tend to act on an aircraft's skin (at the rivets) much like bending an aluminum can back and forth; eventually it breaks. I am sure this was just a surface (paint) crack but they do have to look at every one.