Usernames are online identifiers:
A non-exhaustive list is included in Recital 30
An individual’s social media ‘handle’ or username, which may seem anonymous or nonsensical, is still sufficient to identify them as it uniquely identifies that individual. The username is personal data if it distinguishes one individual from another regardless of whether it is possible to link the ‘online’ identity with a ‘real world’ named individual.
How exactly do you trivially remove all references to the physical, physiological, genetic, mental, economic, cultural or social identity of the person posting?
This is the bar you’d have to clear to ensure someone’s comment history were anonymized per GDPR, miss a single one of these factors and your anonymous data is now reversible and thus infringing.