I would say if they waited until someone had started speaking to leave, then yes, it's rude. However, I think there are circumstances under which it wouldn't be rude.
For example, I went to an event very similar to what you described. It was the club's first meeting of the year, and their flyers said it would be at 6:00, there would be pizza, etc. I showed up at 6:00 and everyone was just eating and socializing. I figured they'd probably start talking soon after, within 10 minutes or so, so I got myself some food and talked to a friend. Well, by 6:35, no one had started talking or indicating they would soon. In fact, a guy with a guitar actually started playing and singing (this isn't a music club and he wasn't a club officer or anything so IDK why he did this) and everyone was watching him play. He kept going on and by 6:45, I basically decided I'd waited long enough and I didn't have time for this because I had other work to do. My friend agreed and we left. Had someone started talking or indicated they would be momentarily, I would have stayed. I felt bad for eating and leaving, but I think if they said the meeting was 6:00, they should have actually started at that time or indicated it was a "pre-meeting social" or something.