We hanged people at Nuremberg for incitement to genocide. Genocide is a crime with a very specific meaning. Yes, bad-faith actors can abuse a law prohibiting incitement to genocide, but the same can be done with any law.
Advocating for genocide is not free speech - it's attempted mass murder. Two people talking with each other and conspiring to kill someone else isn't protected speech - it's just conspiracy to commit murder. And if plotting to kill one person isn't protected, plotting to kill thousands or millions shouldn't be protected either. These people are plotting to commit genocide, and their intention is to use the power of the state as their murder weapon.
We need to prosecute attempted genocide and conspiracy to commit genocide as vigorously as we would any plot to kill any individual. But we have this weird blind spot where if someone plans to commit murder on a large scale using the state as the murder weapon, that somehow we don't recognize it as the same fundamental crime. Murder is murder. Killing is killing. Conspiracy to commit murder is conspiracy to commit murder. Whether the weapon is your own bare hands or the apparatus of a nation state. Advocating for genocide is nothing less than conspiracy to commit genocide.