That's kind of odd. What does the First Amendment have to do with libel law? Doesn't 1A constrain only the government from prohibiting or forcing speech?
A crude analogy, if you stab somebody, the crime is not the knife, it is the stabbing. No need to put a restriction on knives (although there's some but you get the idea).
Originally, libel was considered completely devoid of 1A protections. Most defamation laws even allowed you to sue for *truthful* statements. However, in the landmark case New York Times Co. v. Sullivan (https://en.wikipedia.org/wiki/New_York_Times_Co._v._Sullivan), the USSC reversed this and ruled that the 1st Amendment partially applied to libel and laws must be scoped as such.
That's unnecessary, but probably you need to retake such class.
New York Times Co. v. Sullivan, if anything, established new criteria intended to further protect the rights given by the 1st Ammendment; among those the so called malice standard that I referenced in my earlier comment.
They can't punish it either.