Hmm yeah that's true... So really the question is who decides what "sufficiently anonymized" actually means. Or what counts as personal data and what does not. Probably only a court can answer these questions since the GDPR is not very precise in that regard

I guess the best way to find out is to request deletion of all data including comments and posts, and if they don't comply then take them to court or file a complaint with your national Data Protection Authority