Sportswriter Bill Simmons just did an interesting column trying to break down which movies were sports movies and which movies were dramas with sports in them. One of his defining criteria was: if the sport was removed from the story and replaced with something equally dramatic would it still work? I just finished watching Diggstown, which is a con movie which culminates in a boxing match, and I can tell you: this is definitely a sports movie.
Although the specifics of Diggstown's plot are a bit complex – as befits a con-movie – all you need to know is that James Wood has bet Bruce Dern a lot of money that his friend, an over the hill boxer who never amounted to much, can beat any ten men in one night in a boxing ring. Dern knows two things: one, that is such an insane bet that he has to take it; and two, that anyone making a bet that insane clearly has cards up their sleeve, which means he's definitely being hustled.
Boxing is the perfect sport for these sorts of shenanigans. It's the easiest sport to fix because it only involves getting a single person on the take, but it's also the sport with the highest dramatic stakes – someone could easily get beaten to death in the ring. That means that the fights can easily be worked in any direction the script needs them to go – they can be gimmicked in favor of the con-artists, but they can also turn into deadly serious situations at the drop of a hat. Furthermore, since there are ten fights there is a lot of room for surprises. Because it's a movie you can probably predict that it won't end unceremoniously in the second round, but there are so many variables involved that there's still a lot of inventive ways for things to go sideways.
So, yes, this is a sports movie. It wouldn't work without the boxing; I can't think of anything else you could sub in and still get the story to work. But even people that don't like sports movies will like this movie because the con parts are so well done. Woods makes a good smart-ass – always a plus in movie hustler – and they do a great job of keeping Dern's threats just realistic enough that the stakes are high but they almost never become so serious that the more comedic parts of the movie seem out of place. The fact that there's half an hour of fights in this movie is merely a bonus.