Using the --backup parameter enables the delta-transfer algorithm, so the use of the -u parameter is not necessary.
How can we understand
--backupimplies the delta-transfer algorithm?
-u, --updateskips files that are newer on the receiver. The delta-transfer algorithm is to actually transfer a file, but not to make a decision whether this file will actually be transferred or not. So how does enabling delta-transfer algorithm imply
Note that I found neither of the two are mentioned in the manpage of rsync, but I might miss something.