I think that making the twig tag as synonym of theming is not suitable. There are many issues related to twig as it is, not related to the theming concepts of drupal 8.
I think that questions about using Twig in Drupal are different enough from general theming to warrant removing the tag synonym. Based on the Twig-related questions I have read, most were about "how can I do this Drupal thing in Twig (eg, absolute links)" and not "how do I iterate in Twig" or "how do I style this Twig output".
While Twig is most closely connected with the themes
- modules can provide Twig templates
- render arrays can contain inline templates
- Twig tokens appear in few odd places (like in Views)
Those are all output related, but I wouldn't consider them a theming task.