Contacting the admin should be the right thing to do.
Here’s an explanation from the forum software:
Users can’t be deleted if they have posts. Delete all posts before trying to delete a user. (Posts older than 60 days old can’t be deleted.)
And in this case there are older postings.
Btw:
Anonymising is in my oppinion absolutely compliant with GDPR. If there is no relevant personal data in the postings, they can not be linked to someone later on.
It’s like with statistical data once they are anonymized.
Of course it is the responsibility of the user in the first place to take care not to publish some personal data.