I emailed the chairs anonymously. I did not reveal the author's name. Just hope that they can rethink/modify this anonymity policy in the future.

They replied "thank you".

A malicious reviewer can help the author to know other reviewers -- I think this is what happened in this case.

No, I did not contact the meta-reviewer (maybe the meta-reviewer himself is malicious? He told my name to the author). Since the author already knows me, if I do anything he would know it is me. What will you do in this case?