Twilight was surprised. Why hadn't she thought of that? She had been working under the assumption that Sunflower had been normal and "something happened" to take her magic away, and now she had confirmation. Whatever had happened to Sunflower had obviously happened after her cutie mark had come in! It wasn't much more of a lead than she already, but at least it was something.  It meant the mare had obviously had magic at one point, and then lost it. She couldn't have been born without it, or she'd have never received her mark!