Now i dont want to defend reddit here, but afaik most comments are not subject to GDPR as long as you dont know they contain personal data and they have been detached from other personal data fields (like username).
So by removing personal data fields, they most likely become "anonymized".
Of course thats not the end of it, you have to consider the available technology to de-anonymize this data for it to be legally called anonymized.
But i dont think there has been any case where this was challenged before.. and i bet most supervisory authorities would discard such complaints as being "too hard to follow through". (i got that reply from the Netherlands authority for checking newsletter opt-in from a website)
And i certainly dont think reddit or any operator will be forced to delete comments because they could be deanonymized depending on the content the user wrote, when most comments probably cannot be deanonymized.
Having to check everything for potentially identifiable data in that regard would be ridiculous for website operators.
Maybe some light checks sure, but not as deep as it would be required to truly anonymize everything that a user could have written to identify them.
Alot of that information becomes fragments as soon as you unlink it from the user. e.g. 12 people in a post wrote "I am gay", great. But if you cant link that back to other comments of the same users somewhere else, its not identifiable, just text.