Override username theme

There ist this Setting-Description:

"Override username theme": If this option is selected, the standard username theme function will be overridden to use the "Real name."
here: https://www.drupal.org/documentation/modules/realname →

I guess this function will replace the common Username-Field (f.e. during Registration), but it is not implementet in the D8 version so far, I'm correct? Or do I missunderstand the function and the user has to register nevertheless with username?

Thx.