The author of that article is also the author of the Goji framework (very nice web framework for Golang): https://goji.io/
I understand it fine except this bit right at the end:
var _ http.CloseNotifier = &fancyWriter{}
var _ http.Flusher = &fancyWriter{}
var _ http.Hijacker = &fancyWriter{}
var _ io.ReaderFrom = &fancyWriter{}
It allows you to make sure fancyWriter implements all those interfaces, so that any future refactoring that breaks compatibility breaks at compile time.
You know how Go makes interfaces concordance implicit ? Whatever has a Read() method is a io.Reader without asking for it ? This explicitly tells the compiler (and other developers) the same thing.