it is false:
Backend Port is the port of the Apache server.
On Cloudways managed servers, Apache works behind Varnish and uses port 8081.
- cloudways.com/blog/magento-2-varnish-configuration#magento-2-varnish-configuration
- web.archive.org/web/20240630231720/https://www.cloudways.com/blog/magento-2-varnish-configuration#magento-2-varnish-configuration
Other websites spread this false information, e.g.:
Cloudways uses port 8081 for Varnish