Sure, he didn't use the verbatim phrase "Fans made Serenity happen," but his impassioned thanks seemed to imply that fans had something to do with it.
I think he was trying to suggest 'It's your movie', as you it's a movie for you and you can make it a success. I could be wrong.
From the intro that ran before the preview screenings of Serenity:
Hi I’m Joss Whedon. Before we begin this special screening, I have a little story I want to tell you. It’s about a TV show called Firefly. Firefly went on the air a few years ago and was instantly hailed by critics as one of the most cancelled shows of the year. It was ignored and abandoned, and the story should end there. But it doesn’t, because the people who made the show and the people who saw the show—which is roughly the same number of people—fell in love with it a little bit too much to let it go, too much to lay down arms when the battle looked pretty much lost.
In Hollywood, people like that are called unrealistic, quixotic, obsessive. In my world, they’re called Browncoats. Now whether you watched the show on TV or saw the DVDs—or whether you never set foot in the Firefly universe before tonight—the fact that you’re here means that you’re part of something, something that is a little bit remarkable. This movie should not exist. Failed TV shows don’t get made into major motion pictures—unless the creator, the cast and the fans believe beyond reason. That’s what I have felt, and that’s what I have seen in the DVD sales, the booths at the cons run by fans, the web sites and the fundraisers—all the work the fans have done helped make this movie.
It is, in an unprecedented sense, your movie.
[from [link]
[there's more, which I snipped for brevity and other people reading this thread who don't give a shit]
Yet we all sit and go 'Yeah, we made that movie happen!'. Uhm, no, we didn't. At all.
But it's amazing all the fans think we Done The Impossible and made that movie happen.
Maybe Joss shouldn't have actually said those words, then:
Because remember: they tried to kill us. They did kill us, and here we are. We have done the impossible, and that makes us mighty. Thank you for helping getting this movie as far as it has gotten.
[from the same link]