Post something personal, then put in a 1st GDPR request and ask for all the servers (and their jurisdictions), all the backups, all the 3rd parties with access related to the post itself. Then ask for the security certifications and GDPR policies of each entity named. Then ask the company to delete the post for you in a 3rd GDPR request.

I don't see how they can argue that they own the data, and then turn around and say they have no way of making sure there is no PII - if a user thinks that a comment or post contains their PII and delete it, then either the user owns it or reddit needs to respect that is was deleted for that reason