I've spotted an error in the documentation. The varnish service is disabled by default, so the command to restart wouldn't be causing any effect. In order to actually enable it, the file
/opt/bitnami/varnish/scripts/ctl.sh.disabled needs to be enabled first:
sudo mv /opt/bitnami/varnish/scripts/ctl.sh.disabled /opt/bitnami/varnish/scripts/ctl.sh
Then continue the instructions from the "Restart Varnish" step, and ensure your site is reachable using the port 81 before switching varnish to use 80 by default.
I've just followed this instructions (apply the change to enable the varnish script) in a fresh WordPress Pro installation, and it seems to be working correctly:
Looking forward to your feedback,