"Propaganda" and "documentary" are two entirely different things.
One reason to doubt that "2000 mules" is a documentary is that much of what looks like it's real actually isn't.
In an email to The Post, Phillips said that “the movie graphics are not literal interpretations of our data.”
I haven't checked myself if the first frame is an actual movie still, but if it is, that's a pretty good illustration (and I remember that image flipping technique from the trailer):
Another WaPo article is a transcript of an hour-long conversation with D'Souza being pressed on what the actual evidence is, and D'Souza's discussion strategies felt familiar to me (and probably do to regular readers of Metabunk).
D’Souza admits his movie does not show evidence to prove his claims about ballots being collected and submitted.
"The movie is driven in part by decisions about what makes the movie work well."
"2000 Mules" is not a documentary, because it doesn't reliably document what it is claiming: you never know if what you're shown is real or a mock-up, and it omits what would be crucial evidence for the claims being made.
"2000 Mules" isn't about presenting facts, it's about presenting a conclusion, and that makes it propaganda and not a documentary.