How else could they do it? All systems I've seen that have relied on files on a file system use the file name as the "primary key"
I believe you can use journalling to detect file name change events but I don't think it scales well
I believe you can use journalling to detect file name change events but I don't think it scales well
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.