The answer depends on how you have setup ActiveBackup, as you can run it in three different ways:
1) Rsync shell mode via SSH
2) Rsync module mode (directly)
3) Rsync module mode via SSH
When using mode 1, ActiveBackup will login to the Mac and start up rsync itself. This does not go through launchd at all. I can see that you have been advised to use "--rsync-path", but that is not an option for you, simply because ActiveBackup does not let you set this parameter.
Instead what you need to do is to ensure that when ActiveBackup starts rsync on the remote server, it will start up the correct version - WITHOUT the possibility of changing which command ActiveBackup sends over. There's two ways of doing this depending on whether your ActiveBackup version is the one that issues the "rsync" command or the one issuing the "/usr/bin/rsync" command.
If it is the first one (try this first), then edit the /private/etc/paths file and make sure the "/usr/local/bin" line comes before the "/usr/bin" line.
If it is the second one, then use these steps:
Reboot the Mac in Recovery Mode
Open Terminal and disable SIP: "csrutil disable"
Reboot and open Terminal
Move away old rsync: "mv /usr/bin/rsync /usr/bin/rsync.orig"
Link in new rsync: "ln -s /usr/local/bin/rsync /usr/bin/rsync"
You'll want to test this with ActiveBackup, and then reboot into Recovery Mode to enable SIP again with "csrutil enable".
Note that a subsequent macOS update might replace rsync. If that happens, just repeat the process. Note that as Apple hasn't updated rsync for years, it is not that likely to happen.
If you're using mode 2, ActiveBackup communicates directly with the rsync daemon, and does not start the rsync command itself on the Mac. In this case you cannot use the advise of using "--rsync-path", as it doesn't apply here at all. How to fix it depends on how you have actually started the rsync daemon. As you haven't detailed anything about starting it, I assume this means that you haven't actually started it.
However if you did follow a guide to enable rsyncd - you have probably done it through adding a launchd "LaunchDaemon". In that case you'll need to change your launch daemon file and find the key "Program" and change the corresponding string element to:
If you're using mode 3, you'll want to follow the guide for mode 1 as it is the same steps necessary.