Surely that statement admits the possibility that some words are 'violence'. It just specifies that the signatories think that harm is a necessary cost of the s

Surely that statement admits the possibility that some words are 'violence'. It just specifies that the signatories think that harm is a necessary cost of the speech. If the signatories instead thought that words were incapable of violence, there would be no worry of speech which is "harmful to some". I would also argue that that view severely downplays the power of speech, both for good and evil.