From the list you mean?
Every candidate writes "this is based on my second year summer paper...blah blah blah." So there is no signaling value. Overall, I think someone is trying very hard to promote some of the kids here. In reality, I would choose MIT, Stanford, Wharton and Chicago kids over these so-called stars ten out of ten times. The gap in quality between their publications and road papers is astounding. We had a couple of these out for a visit.
Letters generally say the role the student played in the paper. At least a couple of these were the student’s idea. That type of thing is often easy to document (e.g. it goes back to a PhD seminar paper).