Well then there's no nicer way to put this, you're utterly naive about how political and legal systems work the whole world over. I'm not arguing for immunity for politicians, if I had my way all politicians would be launched into the sun (though they'd be quickly replaced by yet more politicians).

I'm arguing that a law like you're proposing will inevitably be enforced unequally, be enforced against people who spoke unpopular truths that powerful people claimed were legally lies, and will be used by powerful interests to suppress their enemies. This isn't pessimism, this is a "this has happened anytime this or anything similar to it has been tried" and anyone with the most basic understanding of human nature could see that.