I totally agree with you. Argo is a feature film, not a documentary. If Argo had been shot as a documentary, chances are very few people would ever have seen it. The fact that it was such a well made and entertaining piece of work brought this story to the attention of the public in a way that no story in a newspaper, magazine article or poorly made action flick could ever have done. Wonder what subject Mr. Affleck will tackle next? Can't wait.
All I'm suggesting is that although Argo and most other historically based movies don't sell themselves as documentaries, the unfortunate reality is that they have significant influence on ordinary people's perceptions of important people and events. This is exacerbated when the director of said movie goes on record touting the accuracy of the film.
Perhaps I can shed a bit of light on this specific point. In my research (wading though a LOT of books and journal articles about historical feature films, along with watching/listening to what must be hundreds of hours of extra features, interviews and film commentaries), I've found that when filmmakers (and I include the whole crew here, not just the director) speak about accuracy of an historical feature film, the bulk of their focus is on visual accuracy--costumes, sets and so on. They want to get "the look" right (let's leave aside how successful, or not, they may be). A lot of time is spent explaining and demonstrating just how close to the real thing the film looks like. An actor's ability to personify a real person. The use of period music to set the proper mood. Factual accuracy is important insofar as it does not interfere with the cohesion of the narrative (actual history is a lot less cohesive than films, or even books, make it seem). But one thing that is almost always overlooked is how the characters speak and behave towards each other (this is more problematic the further back in the past you go). You can tell a lot about the seriousness filmmakers apply to historical authenticity by paying attention to how characters behave and speak. But even then, reality is sometimes set aside so as to convey the spirit of historical truth, at the expense of the letter of it. As anyone who has watched Deadwood knows, there is an astonishing degree of foul language (the opening scene of the series alone had me running to my bookshelves looking for any trace of evidence to support such language in that time period). It is almost entirely anachronistic but it is also, paradoxically, a lot closer to the historical truth than a literal transcript of language usage of that time would appear to a modern audience. If the makers of Deadwood had been faithful to the letter of the language of that period, it would have appeared to the audience that the population of towns like that were rather more polite than they actually were. Instead, the creators transposed the language so as to properly shock, rather than bemuse, the audience. This is a case where revisionism served a quite useful historical lesson--mining towns on the frontier were not populated by polite society. In any event, as most makers of historical feature films are not historians by training (and only a rare few choose to self-identify as historians, like Oliver Stone), the focus on visual accuracy by artists working in a visual medium is understandable, if sometimes a touch irritating to those who work in the field professionally.