Yikes, that's false and not nice. We delete comments for people nearly every day. It's true that we don't allow wholesale deletion of account histories, because that would gut the threads the account had participated in. But we also don't want anyone to get in trouble from anything they posted to HN.
The idea is to balance the need to preserve history (community interest) and the need to protect users (individual interest). We don't get there perfectly, but I can tell you for sure that the vast majority of people who ask us about this end up happy with how we help them.
My understanding of the policy matches the parent comment from communications I've had. Is this a recent policy change?