The response to MailChimp's letter is distasteful at best. I don't know how MailChimp found out about Luke's work, but I imagine having a Dev contact someone infringing on their work is not normal procedure for any company. Lawyers are paid to do this professionally, however Ms. Danin's letter goes above and beyond, illustrating the issues they have and their reasoning behind it. They could have had court documents (in the US anyways) show up at your door, or the site / files could have been forced offline by working with the host.

Your response reflects poorly on you. I would say it was fair to mention your employer as it's listed on your public github profile, and it isn't clear that your employer isn't aware. Additionally, I'd imagine you'll be receiving more removal requests from the other sites you replicate.

Also, releasing your files under the MIT license prohibits you from restricting them to "educational purposes", at least to my understanding, but I could be incorrect here.