Also IANAL:
To my knowledge, it is a bit of a complicated topic. But in general the definition of personal information in the GDPR is by design kept really broad.
https://www.gdpreu.org/the-regulation/key-concepts/personal-data/
https://commission.europa.eu/law/law-topic/data-protection/reform/rules-business-and-organisations/dealing-citizens/do-we-always-have-delete-personal-data-if-person-asks_en

I think the moment your account is still active, all your comments can be identified as personal data by your username and falls under GDPR for sure. If your account is deleted/banned it is anonymized so it could only fall under GDPR if the comment contains identifyable information. But anonymization must be irreversible, so I think reddit is not allowed to have any data linking your comments to each other after you deleted your account.

Now what if you edit your comments/delete them and delete your account afterwards (which schould anonymize it).
If they restore your former contents, doesn't that mean that they still have an identifier for your data linking the deleted GDPR protected data?
I don't think EU looks too kindly on stuff like that, they can be pretty strict for consumer data protection and using data like this seems pretty against the original intention of the GDPR.
The Reddit TOS can also say all kind of stuff, but it won't stick if it breaks a higher law instance.

Spez is a person with no foresight and a bad CEO, i think reddit is about to fuck around and find out what happens if you ignore the GDPR.
I don't think there has been a case like this before, where a social media so blatantly misuses consumer data. This might become a pretty big thing and hopefully clarify the rights of the users in the future.