How to retrieve view row attributes when in twig template
The views-view.html.twig template is at a higher level. To manipulate data at that level, you'll need to use template_preprocess_views_view().
You used template_preprocess_views_view_unformatted() which allows you to manipulate the data in views-view-unformatted.html.twig. This seems like the correct approach given your case.
Here are the contents of views-view-unformatted.html.twig from the views module in core.
{#
/**
 * @file
 * Default theme implementation to display a view of unformatted rows.
 *
 * Available variables:
 * - title: The title of this group of rows. May be empty.
 * - rows: A list of the view's row items.
 *   - attributes: The row's HTML attributes.
 *   - content: The row's content.
 * - view: The view object.
 * - default_row_class: A flag indicating whether default classes should be
 *   used on rows.
 *
 * @see template_preprocess_views_view_unformatted()
 *
 * @ingroup themeable
 */
#}
{% if title %}
  &lt;h3&gt;{{ title }}&lt;/h3&gt;
{% endif %}
{% for row in rows %}
  {%
    set row_classes = [
      default_row_class ? 'views-row',
    ]
  %}
  &lt;div{{ row.attributes.addClass(row_classes) }}&gt;
    {{- row.content -}}
  &lt;/div&gt;
{% endfor %}

You can see that if you loop through the rows variable, each row has an attributes property - the classes you added in template_preprocess_views_view_unformatted() will be part of it.