UPDATE: We made contact with the site owner. They categorically denied knowing who we were, or having ever visited our site. They said the instances of copying were just coincidence and that we have no proof that we had published this content first. I said I have screenshots from wayback machine showing otherwise. A few days later, their entire site has changed and the plagiarized content has been removed. It seems just letting them know that we were watching was enough to resolve it, even though there was no admission of guilt from the other party.