Horness Roe. A. (2013, Animated Documentary) argues that documentary authenticity is “deeply related to the notion of realism and evidence of real events in documentary images”. There is an indexical relationship between documentary images and reality. If the images are evidence of real events and conform to the concept of realism, then the documentary is real.
I guess the author wanted to convey that the subject matter and concept of the documentary was to be drawn from evidence of real events. But it doesn’t have to be expressed in exactly the same way as the real event, the documentary can use a variety of perspectives and techniques to present the real point of view and content. A documentary is authentic if it is based on the public perception that the content is historically accurate.