The text can indeed be changed on your site, either via Manage authentication or using the language customisation feature.
The reason for changing the language string in standard Moodle (MDL-53173) is for everyone who downloads Moodle in the future, and also for translators to know to simplify the text in their language. (https://moodle.org/login/index.php has simplified text in English, but all other languages still have the long text.)