I received a response from Reddit pretty quickly after submitting it. The response told me that I must delete all of the posts and comments beforehand. I'm pretty sure this is in violation of both GDPR/CCPA as it might be physically impossible for a user to delete, say, one million comments. Of course, this ignores the fact that Reddit already restored all of the data that I've deleted.

No tool from reddit allows you to delete all your comments. Reddit doesn't allow you to map all of them. You can check by yourself by searching manually in a search engine for your username and reddit.com. You will see a lot of your comments which are not shown in your comment interface.