If HN removed their record of the email address associated with a username, might that satisfy GDPR? The personally identifying data has been "forgotten". From that point on, the comments could have been entered by "anyone".
I accept that if someone data-mined every comment by said user, they might be able to build a picture of said user clear enough to identify them (e.g. posting times might indicate likey country of origin). Possibly, depending on the content they posted.
(I'm just thinking around the problem. I'm not a security/privacy researcher designing systems I'd like others to use, just an interested user curious where the lines in the law lie, and also what the threat models might be to me as a user.)