To label a movie "non-fiction" is not necessarily to say that it contains no fiction or has no traffic with fiction. On the contrary, the documentarian may make or use fiction as a major part of a plan for achieving unambiguously constative purposes. In such cases, the author is guided by a plan consisting of multiple, hierarchized intentions regarding the illocutionary effects that the work is destined to have. This underlying complexity need not detract from the work's resolutely assertive force and hence non-fictional status.
I do not deny that real audiences can have real trouble deciding whether a given work is fiction or non-fiction. Everybody knows that it can be difficult to tell the difference, especially in these days of dramatic re-creations, simulations, "docudramas," and "fact-based fictions." Hence a sophisticated observer, fairly knowledgeable about (a) the range of stereotypical features currently attributed to cinematic fictions and non-fictions, (b) a particular work's properties, and (c) the context in which this particular work was produced and/or exhibited may still find that judging its illocutionary force requires making a tough call -- one in which she may have less than complete confidence. But I strongly doubt any doctrine proposing that innovations and historical developments show the boundaries between documentary and fiction to be prone to subversion, to be fuzzy and in flux, or to be beyond hard and fast definitions. 1 Styles, techniques, and formal structures typically or normatively associated with documentary status may change over time, as may audiences' expectations that a given property normally indicates an author's assertive intention. But come what may, it is nonetheless the case that a cinematic work or part thereof is non-fiction if and only if an agent makes or uses it with expressed, effective constative intentions.
On the other hand, movie fictions will continue to result from effective fiction-making intentions, the nature of which I shall discuss below. And a genuine hybrid -- a work mixing fiction with assertion -- is one resulting