XKCD: “I just legit adverbed ‘legit’…”

I’m not sure that counts as a legit adverbing of “legit” though, because if we assume that “legit” is short for “legitimately” then it would already be an adverb. Although it would be if “legit” is interpreted as short for “legitimate”. Anyone have intuitions about how legit this is?  

Hovertext: Not to go all sentence fragment on you.