A couple of suggestions:
1. Check with lawyer.
2. Review your contract for the license. Specifically if you define where your blog resides. Medium also clearly states "You’re responsible for the content you submit to Medium and assume all risks associated with it, including anyone else’s reliance on its accuracy, or claims relating to intellectual property or other legal rights. By posting, you represent that you own or have the necessary rights to post the content on Medium, and that doing so doesn’t conflict with any other licenses you’ve granted."
3. If your claim is correct, NG probably just made a mistake thinking "oh medium.com, we didn't allow that!", however if you contract very specifically dictates where the blog resides (ULR for example) they are most likely negligent. We can't let content creators simply throw out DMCA notices left and right without them doing their homework:
Think Long and Hard Before Filing a DMCA Notification or Counter-Notice
Please think carefully before submitting a claim or counter-notice, especially if you aren’t sure whether you are the actual rights holder or authorized to act on a rights holder’s behalf. There are legal and financial penalties for fraudulent and/or bad-faith claims. Please make sure you are the actual rights holder, or you have a good-faith belief that the material was removed in error, and that you understand the repercussions of submitting a false claim.[0]
[0] - https://medium.com/policy/medium-terms-of-service-9db0094a1e...
[1] - https://medium.com/policy/mediums-copyright-and-dmca-policy-...