The FCBIS _stats member should be protected by the mutex, as indicated in the header comments. However, it can be written to in an synchronized way by the fetcher callback created in extendBackupCursor here:
https://github.com/10gen/mongo/blob/cbb2dd5bbf0407792b3a808bd8adc131e6fb9b51/src/mongo/db/modules/enterprise/src/fcbis/file_copy_based_initial_syncer.cpp#L959