No, while there are Constitutional limits (especially when the injured party is a public figure, see NY Times v. Sullivan), deliberate and often also reckless, harmful falsehood is not protected speech.
> Does this set a precedent that anyone who spouts conspiracy theories about an event is targetable by those who suffered from that event?
No, it just reinforces the existing precedent that people who deliberately or recklessly spread falsehoods about people can be sued by those injured by the falsehoods. Defamation isn't a new thing in law.
And this isn't the first time that has bitten Alex Jones.