adding firstname and lastname on registration

The user_register action hook only passes 2 parameters and your callback requires 3, which will result in a PHP fatal error. You should instead be using the insert_custom_user_meta filter like so:

class FootballPoolRegisterNames {
  public static function init_extension() {
    ...
    add_filter( 'insert_custom_user_meta', [ __CLASS__, 'insert_custom_user_meta' ], 10, 4 );
  }
  ...
  public static function insert_custom_user_meta( $custom_meta, $user, $update, $userdata ) {
    $custom_meta[ 'first_name' ] = sanitize_text_field( $_POST[ 'first_name' ] ?? '' );
    $custom_meta[ 'last_name' ] = sanitize_text_field( $_POST[ 'last_name' ] ?? '' );
    return $custom_meta;
  }
}