You‘re right, if the law was applicable then they‘d have to „process“(delete) the data.

But since the right to information weighs heavier than the right to be forgotten (except when it comes to personal data, which can be used to identify a user) Reddit is not required by the GDPR to delete posts/comments that do not contain such information

But we can‘t really know for sure what counts as personal data unless someone drags a company in front of a court over something like this