The authenticity of a documentary is ‘deeply linked to notions of realism and the idea that documentary images are linked to notions of realism and the idea that documentary images bear evidence of events that actually happened, by virtue of the indexical relationship between image and reality’
Horness Roe. A. (2013) Animated Documentary. Basingstoke: Palgrave Macmillan.
I think the author wants to make a point that the authenticity of documentaries is strongly linked to realism, that the images and sounds of documentaries carry the actual happenings as a testament to real life. However the images of the documentary must be authentic and valid, because the authenticity of the documentary needs to be corroborated by the images as well, and therefore the validity of the images must be proven.