Who is "they"? Are you asking about rysnc developers or filesystem developers?

For a filesystem the developers are free to innovate on any form of filesystem-wide block checksumming and content-addressed deduplication approach. This kind of global block-level content awareness is entirely decoupled from filenames and paths.

Using rsync to underpin a filesystem strikes me as a gross hack done by developers punting on the real challenges of modern filesystem development.

WRT rsync doing better than --fuzzy, the options are more limited since it's a userspace tool working within the confines of filesystem APIs like POSIX.