Both 'libel' and 'liable' are real legal words.

Libel refers to written/printed/online defamatory statements [1] - as opposed to slander, which is spoken defamation.

[1] https://en.wikipedia.org/wiki/Libel#Libel

I believe declan is referring to the fact that libel, being a civil tort, exposes one to be liable for defamation, not guilty of it (being that it's not criminal).

Yep. My point is that criminal libel no longer exists in the United States, so you can't be "guilty of libel."

You can held liable for libel, but that's an awkward phrasing, so "liable for defamation" is broader and better.