Is it taboo to disclose info to 3rd parties about the review status of a paper of other authors?
To me, their response was very unprofessional. I'm assuming that the review is still blinded and further communication with the authors directly may not be possible or even useful. 

As a peer reviewer, you are a volunteer for the journal. If the lack of professionalism of the authors made you uncomfortable, you could communicate this with the editor. Although the editor might not do anything about it, he or she might reconsider publishing future papers from authors who do not respect blind peer review. Publishing our work is a privilege, not a right. I am not an editor, so I'm not sure how editors may handle such a situation. Although this case involved the reviewer and not the authors, PLOS ONE received attention a few years ago for removing a reviewer who made a sexist remark in their review: http://www.sciencemag.org/news/2015/05/plos-one-ousts-reviewer-editor-after-sexist-peer-review-storm