And unfortunately @dan's expectation is not what is always happening as exemplified in my case.
Has @johnsarris edited his post? It doesn't look anything like yours.
But even if commenting isn't effective in every case, it will still help tremendously if the evidence of plagiarization is just below the upvote button. Most (real) users are too lazy to do any kind of check for the content, but can easily see the top comment and abstain from upvoting.