Was it really a show about nothing?

Nope, never been.
They just thought that's a funny, clever, artsy concept, but they certainly didn't deliver it, nor had any intention of delivering it.

This show has clear rules, topics, story arches and a general premise.
So not only it's not about nothing, it's clearly about many things.

No it was never about nothing.

It's been decades since I've seen this but I remember there was a bundle of episodes where there was going to be a new TV show based off of Seinfeld and his friends... They were trying to come up with ideas for the series (an antique shop came up) but when they were in front of the big wigs, George blurted about that the show was going to be about absolutely nothing (where nothing at all would happen during the show). Somehow that translated to the masses that the Seinfeld show was about nothing which was never the case.

I've said this before on here but people should just say nothing interesting. Cause for a show to be nothing it'd just be a blank black screen for 22 minutes. And nobody would watch it. The stuff George talks about when he and Jerry are doing a show should just be called not interesting. Like getting up in the morning, taking a shower, eating breakfast, and then driving to work is not interesting. But it is certainly not nothing. Actually, some people might like watching someone take a shower and that'd be the most interesting thing of all that.

It's the go-to sitcom for people who think they're too intelligent to watch a show like Friends.

It was a show about whining.

It was a show about little things; these little things were mundane concepts of life that get spun out of control due to character's hang ups about dealing with said concepts.