can someone explain how this is true?

This statement assumes that communication with other human beings is a fundamental human right. Once you stipulate that axiom, the speaker and the hearer (if any) have the right to transmit and receive information, respectively. While that is a single denial of the right to communicate, the author counts the number of persons involved in that denial, and concludes that it is a double wrong.

However, there are monastic and religious orders that consider silence a virtue, and consider it a sin to interrupt another person's vow of silence. That is an exception that proves the rule.

If a man has a right to speak, it automatically follows that man another has the right to listen, as speech automatically assumes communication of ideas.

Depriving the one of speech deprives the other of listening and learning if he so chooses. Hence, it is indeed, a wrong, greater than a double, as many people listen to the same speaker.

If you are not able to provide necessary information then people can be taken advantage of. If we can not speak up, we are being controlled by the ones who are withholding free speech. People need to know things, if it wasn't for free speech we would be lost. Injustices would always be carried out in silence and we would never know about them, etc etc