The key feature of Time Machine is hard links to directories -- which is only possible on modern HFS+ (and rsync doesn't even try). Some people like the UI too, of course. Without hard linked directories, a full --link-dest backup of a decent sized disk, with zero file changes from the previous pass, can easily consume 100MB (and take 45 minutes to perform). This disk consumption might seem insignificant, today, but…
The --link-dest option of rsync, used in this project, actually creates hard links.
...to files, but not to directories, which is the key point.