Wow, Carlo, sure looks like plagiarism to me. Yes, unless you've filed with the copyright office, it's hard to have legal standing. That said, the moment you create something, the copyright belongs to you, period. (If you publish your piece, there's no need to include the copyright symbol as it's implicit.) If it were me, I'd go to the Medium help desk, but I'm an editor, and I don't look kindly on plagiarism.
Btw, I read your piece a few weeks ago, and it was wonderful!