There seem to be 2 fundamental misunderstandings here:
1) That other processes cannot see data written to a file until it is flushed to disk; this is not true; while file data is still in file cache, it is visible to other processes.
2) That rsync writes the file on the destination directly; it does not; it writes into a temporary file and renames that file when it is complete.
While you’re correct; I never made either of those assumptions