I think they mean the show and not the characters. Even in that case, the show really isn't about nothing anymore than other sitcoms like Friends and Cheers are about nothing. The show still has plot lines. My love of Seinfeld is mostly how well the plot lines within each episode are weaved together and how the show invented so many phrases that we now use (or at least I use and most people understand).

The show is so quotable and relatable even almost 20 years later. Friends also does a pretty good job of this, but my problem with Friends is that you're supposed to like the characters (except Ross maybe) and they're all pretty terrible people. Seinfeld doesn't try and hide how terrible its characters are.