That code was not updated for 2.0.
In 1.3.x the USN number was written to the manifest files. In 2.0, we should store it in the database as well as in the dlist files.
My strategy was to grab the current USN, den do the listing, such that changes could not be lost (USN may update after grabbing it, but this will only cause the files to be scanned again later).
There is as call in the backup method that creates the new filelist, and I think that would be the best place to record it.