You MUST NOT reply and make sure you DO NOT communicate verbally, or write any letter and document with/to the other party's lawyer....!!!
You are advised to IMMEDIATELY CANCEL and ERASE the Article from your website...NOW!!!
The reasons for doing the above-mentioned is to safeguard your defense and reserve "Bullets to Shoot Back" if undesirable things should further develop.
You should STAY CALM and BE QUIET on this matter as if nothing had happened for the time being.
Copy infringement is a wide topic to argue and prove, they probably also don't have copyrights on thier own portal and could be wrongfully threatening, as such you should not worry to much at this moment. All other matters, just wait and see what's next.