I think he means something else.
The shelf exams are exams given by many schools composed of retired NBME questions, which are questions previously but no longer used on the Step 1. They are given once to students and are collected afterward.
In the context of Step 1 prep, 'retired NBME questions' often refers to two sets of old questions which were released to the general public over a decade ago, and are out of print but still found on Ebay, etc.