Indeed... It's safe to say that >95% (I would hope) of DC writers were huge fans of the characters/universe before they started writing for DC, so my only thought as to why the "fan fiction" label would be applied was if his writing quality was deemed closer to an amateur than a professional/career writer.
So far, he comes off as an excellent writer, metaphor and exposition coming very naturally to him... He is also able to demonstrate a character's personality and attributes without directly telling us what they are. His arcs are pretty long, so I have yet to see how he ends stories, but I'm really happy with his abilities so far.