They have a public list of their journalists (or at least, their work): http://www.economist.com/mediadirectory/books.cfm and staff can publicly say they work there.
A friend of mine works there (or interns, not sure); she post on Facebook and twitter the papers that she wrote, or the video that she shot, but she also includes other papers that she really liked. I'm assuming the official policy is stricter, but what she does is rather implicit, that only a handful of people notice (and presumably mostly other journalists that help with her ‘beat‘). So far, she has defended, sincerely, every paper that I criticized (I go after The Economist a lot) — most of them quite far from her beat (I'm closer to the Economist's line when it comes to the topics that she covers).
I think the anonymity allows people, who are experts in their field, to be candid but protect their reputation.