they do a lot of hard-hitting journalism - not.

[Maybo]

Right now, I work for a local newspaper called the Nassau Nutshell. You can tell by the name that they do a lot of hard-hitting journalism - not. They put out one paper per week, and I contribute a couple of articles, mostly on local events or parenting advice.

Source: The Perfect Son by Freida McFadden

What does "not" mean?

 

[Tarheel]

It negates what came before. It's like "Not really". The person is telling you not to believe the sentence you just read. In other words, they don't do a lot of hard-hitting journalism.

 

[Skrej]

Note that this usage is very informal, and more typical of spoken English than written. It's meant to express sarcasm. It was all the rage amongst teenagers and young adults in the late 80's and early 90's.

It was popularized and perhaps even parodied in the film Wayne's World (which was based on the SNL characters developed by Mike Myers and Dana Carvey) where it was a catchphrase or running gag of Myers's character. "Pearl Jam is a great band - NOT!"

I haven't heard it used in ages. It's almost a nostalgic throwback at this point. Here's an old article from 2017 discussing it.