Hi David,
Thanks for your very interesting question.
In our opinion comments are indeed no personal data, but sometimes it can be protected by copyright and be linked to the author. Therefor we do not let users delete their comment but this action is a request send to the sites admin. The admin has the ability to delete the comment or (in a future release) make the comment Anonymous through our wp-gdpr backend view. In both cases the user gets an email with the taken steps.
Hope this answers your question.
Kind regards,
Mieke
Disclaimer: We are no lawyers, but WordPress developers that followed some courses on GDPR.