I would have expected
username_as_name setting to apply to all users imported from phpBB - the importer modifies the usernames both for regular and anonymous users. However, this setting currently only applies to regular users, anonymous users always get an empty name.
Besides, wouldn’t it make sense to have “only if modified” as an option here? It would probably be best to set the original username as Discourse name only if original username and Discourse username aren’t the same.