It's certainly true that a 100% objective documentary is rare--likely nonexistent. The point the author of that document is trying to make is that BFC doesn't count as a documentary because it relies upon intentionally deceptive and misleading methods to make its point. Most documentaries present the facts in a way that helps skew the perspective to that of the filmmaker, and that's fine. BFC doesn't simply present things in a certain light to get you to look at the topic in a new way; it lies to you, twisting the truth and coercing you to come to conclusions to which you probably wouldn't arrive under normal circumstances.

The first time I saw it, I thought it was genius. Now I think it's trashy genius.