Have you got a link for that?
The definition I'm reading says "Words spoken over the air on television or radio are treated as libel (written defamation) and not slander on the theory that broadcasting reaches a large audience as much if not more than printed publications", so if it's not being televised, it's not a broadcast that's reaching a large audience.
I studied it as part of my degree,
Slander is the threat of a single publication through a medium incapable of mass dissemination, longevity, or permanence - key word is incapable. If it's recorded it is capable of mass dissemination and it would be classed as libel. http://defamation.laws.com/defamation-laws/libel-vs-slander