If they were that close, they wouldn’t run a site which solely relies on the safeguarding of that data. I cannot imagine they don’t know how to handle and backup data.
As for the gdpr, selling the data to an AI company for LLMs is probably anonymized. Or they have a database that does not contain any account information and only the posts. From a cursory read of the gdpr your personal data is your account, not necessarily your posts. If the posts are no longer associated with an account they are free game to reddit.
Ironically, deleting the accounts might make it easier for reddit to use the data.