Actually, that sounds like the same thing. USN is implemented as a sequence number on NTFS such that you can “list all changes since last ID”, which is what I understand is the same as what FSEvents does.
Now that USN support is there, all infrastructure is ready, so it is just a matter of plugging in the calls to retrieve the list of modified files.
The --changed-files option is planned to be used with a monitoring service. The idea is that “some program” (i.e. the Duplicati server) is running all the time and monitors the source directories. This monitor process then records a list of changed files, and when a backup is executed it inputs this argument to avoid the lengthy scanning process.
However, if the OS supports USN (or FSEvents), there is no need to go through the trouble of using --changed-files and monitor, as the OS does that already.