If ssh accepts the local .ssh/known_hosts it should also accept the local .ssh/config file.
So I can only assume the rsync task enforces the strict version. You could check for the actual rsync command from shell when it is running.
From a security perspective I think the behavior is like it should be. How often does the server change its key? It should not do that too often.
Again, if you really need StrictHostKeyChecking=no, as I said, check the "-e" option of rsync.
Or you could run a cronjob using ssh-keyscan to update the known_hosts automatically.