How do I overwrite a lazy service?
default.services.yml is just basically just documentation/ an example, it's not actually loaded, at least not by default.

To load a services.yml file, it either needs to be named services.yml, or you need to explicitly added to the settings.php file, see example.settings.local.php and how it loads development services.

The redis module also provides implementations for those services, and it does so by providing a separate example.services.yml file that you can either include directly or copy and customize. That definitely works, I wrote and tested it.