Since FormBase includes the StringTranslationTrait trait, all forms in the TFA module can (and should) rely upon the t() method.
So a simple replacement of all occurrences of t('...') with $this->t('...') in the form classes is what's required.
Though this issue is minor, but it is in accordance with the OOP best practices which encourage the usage of class methods instead of global functions.
Since FormBase includes the StringTranslationTrait trait, all forms in the TFA module can (and should) rely upon the t() method.
So a simple replacement of all occurrences of t('...') with $this->t('...') in the form classes is what's required.
Though this issue is minor, but it is in accordance with the OOP best practices which encourage the usage of class methods instead of global functions.
For more information: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21StringTranslation%21StringTranslationTrait.php/trait/StringTranslationTrait/8.2.x