Board Thread:General Discussion/@comment-6697950-20160731102536/@comment-28251580-20161119081834

Yes, it was written by Kawahara himself. Only the Alternative series (GGO, Clover's Regret) and in parts the Mahouka crossovers are from different authors, and obviously the manga and game adaptations. If there is a different author, it is usually written on the article of that story / series.