Board Thread:Fun and Games/@comment-26391060-20140617033900/@comment-1173941-20140617180810

Probably to some extent. I think the definition you'd use is something like, "A popular fan-idea related to canon, but has never been made clear by the author".

Kayaba being chuunibyou would probably be a fanon idea that happens to have a lot of support in canon. The actions he took, and the stuff he said, (if you include the in-character Q&A from the web novel times) point to it. There's also how the 4koma joked about it.

SAO and AW being connected is also Fanon, and so is the idea that Kirito and Asuna used the Incarnate System in Volume 1.