I don't even think you're getting the basics of this. It's not "pleading the fifth", or anything remotely like that.
There's a very high evidentiary bar for defamation lawsuits filed by public figures, and proving that someone was lying is difficult. "I'm too stupid to understand that; it wasn't a lie, it was my own incompetence" is a perfectly valid defence in cases like this. Barring some Emails where the reporter said something to the effect of "I know this isn't true, but I'm going to tweet it anyway" (which could exist, I guess), it's next to impossible to show that they KNEW they were lying.