I am ranking for various sentences from the copied article...
That's what I was referring to, random sentences/snippets. That doesn't matter so much as coherent phrases that a searcher may use. As long as you can still rank for those then the search engines are on top of identifying who the original author is.
Remember, whoever has the most internal PageRank obtains the credit for the contest. Not certain how much importance being first to publish is given. Whatever the case is, whoever wins obtains all the PageRank from the scrapers. Double-edged. Could work against you. Which is why in my original post I advised you to obtain more credibility than the other site with more and better inbound links.
Don't get caught with unnatural links. That'll worsen your situation. You have what you say is expert content. Good. Now go out and tell others about it so they link to it.