I doubt the manager would post the "SOLD OUT" sign if the theater had even a few people already inside, and about to watch the movie. That would only prevent a few more people from buying tickets, and result in the 4 watchers emerging with even more extreme reports of how few people were in attendance. Why would a manager want to do that? It's not as if people assume a movie isn't worth watching if they don't see "Sold Out" signs for it on theater ticket windows. And there's no way a manager would cancel a showing of any movie, if even a single customer was already there in a seat waiting to see it. They'd just cancel the movie's run at the theater earlier than planned.
The guy who reported this may have poked his head into the wrong theater, perhaps a theater where a later showing of the movie was scheduled, with few people having arrived yet. Or he may have looked in the right theater, but long before the scheduled start time, when few people would have already arrived. Many theaters do the bulk of their sales for new highly publicized films via advance and online sales, and Brokeback Mountain is no doubt being attended by a lot of gays, including some sizeable organized groups from colleges who would purchase in advance to make sure no one was turned away after the group arrived.
It is a campaign where the studio buys up all the tickets for the performance online....selling out the theatre. Then next week they can report the highest gross per theatre numbers in the trades.
Miramax was very good at doing this.
The suspicion, I think, is that wealthy gay people are buying up tickets. OF course, if a movie is about to start, you realize no-one is watching it, it might serve a theater owner's interest to make it appear like the movie is a hit, and is succeeding in appealling to a wider audience. Sorta like musicians throwing money into their own hat.
Or...maybe ONE person or ONE group bought up ALL the tickets (online?), but NO ONE really was going to see it? There are a lot of possible scenarios.