Read some history of how that came to be. You might still disagree when you're done, but I suspect you'll at least be able to answer your own question.

No legal system anywhere can be absolute about not determining what the "truth" is, even when it comes to speech. The most common example is yelling "fire!" in a crowded theatre and causing a panic. Or posting a bomb threat to social media. That kind of mischief is illegal in most places as there's actual harm. And the law will have to determine the truth that there was no fire or no bomb, and that your speech had no purpose except to cause harm.

Another example is speech used to commit fraud or extortion. Were your profitable lies to exploit the vulnerable based on a genuine belief, or did you have criminal intent? The law will have to decide, and the situation will not always be clear, and often it comes down to determining intent.

In the case of holocaust denial, in Germany the potential harm of allowing it is seen as more genocide, so they've calibrated their free speech laws in respect of that. But there's never an exact place to draw the line; every legal system is going to have to make such judgment calls.