The "by itself" is doing a lot of work in that sentence. Sure, if you post someone's home address with an allegation that they're a paedophile, the instruction to attack them doesn't harm them, it's people following the instruction. And the bombardment of words sent to harassment victims isn't the sole factor in the emotional state of harassment victims, and it's the virus that kills not the antivax sentiment etc etc.

But they harm people rather more directly than Twitter having the freedom to delete those words if its management feels that would be the responsible thing to do does...