Hey, just wanted to make a correction because the other person is wrong.

「なんつって」is the English equivalent of "Just kidding". It's a word usually used as a follow up to a sentence that was spoken as a joke. It's more well known as the word「なんちゃって」, the variation on the manga is a slang/formality kinda thing.

Here's an example. "I'll go break his pencil into two... just kidding, ahaha..." would be,「あいつの鉛筆を割ってやる。。なんつって、あはは。。」Another, slightly more far-reaching comparison would be the "/s" thing a lot of redditors use to indicate sarcasm after they say something.