Opening a case against Newsweek lets them subpoena all sorts of records that they aren't entitled to with out a case, primarily because the truth is a defence to slander / libel, and if this guy loses then he's forever Satoshi.
Also it's going to be pretty difficult to prove that he suffered damages from the article.
It depends on what the fallout from it is, in terms of internet attention. If he's not "the real Satoshi" and he starts getting harassed because of this article, I'd say he has a pretty decent case: If the statement "is false, caused harm, and was made without adequate research", then most non-celebrity citizens have a pretty strong case.
