I mean, how can you know that he's a liar, i.e. that he believes that what he says is wrong but still says it? Because that's the moral crux of matter, to me.
Ah, ok, I was trying to figure out what the difference is between this case and the thousands of lies that other right-wing figures tell every day, and maybe it's this: never admit that you lied! Or of course it could be that the others have better lawyers, deeper pockets, better connections etc. etc.
OTOH of course, you can't be sued just for lying. For instance, who is going to sue Trump for lying about the amount of US aid for Ukraine (it's not 300 or 350 billion, "just" 119 billion), about it being "given away", and about the US being the biggest donor (the EU has spent 138 billion)? Maybe Joe Biden?
Pretty sure that in the US, the 1st amendment should trump this. It's a laughable concept, anyway, way too dependent on the current Overton window; makes perfect sense to have an article on Wiki "BuzzFeed, Salon and PinkNews are reliable sources" Pedia.