The default version 2.6.9 of rsync in macOS was released in nov 2006. And there has been several fixes and releases since then. The news about the current release of rsync is here. Due to new features in rsync and dependency to shared libraries it is not possible to bundle the latest version together with RsyncUI.
It is adviced to install rsync as part of Homebrew. In RsyncUI select user configuration and set path for optional version of rsync. Install homebrew and install the latest version of rsync as part of homebrew. Execute the command
brew install rsync
to install the latest version of rsync.
Snapshots
RsyncUI supports snapshots of files. Due to a bug in version 2.6.9 in rsync, the snapshot feature of RsyncUI require to install the latest version of rsync.