> it must support everything modern rsync supports to be considered compatible, right?

Apple's openrsync says that it is compatible with rsync protocol versions 27 - 29, which predate rsync 3.X. I don't consider it modern.

It looks like there are quite a few glitches in Apple's openrsync. Deletion of two files resulted in four deletion entries.

    rsync -av --delete --dry-run a/ b/

    Transfer starting: 2 files
    folder1/file2: deleting
    folder1/file1: deleting
    ./folder1/file2: deleting
    ./folder1/file1: deleting

Also, if a directory has extended attributes, `rsync -av --extended-attributes` resulted in `._` folders in the destination directory.

It is not an acceptable replacement for the original rsync.