To get the field object of a group field sub field you need to treat the group field as a repeater, not as an array.
while (have_rows('twi_fields')) {
// always happens once on a group field
the_row();
$text = get_sub_field('twi_text');
$field = get_sub_field_object('twi_text');
$fieldtype = $field['type'];
}