I can't comment on what is legal, but I very much dislike the idea that my comments are the property of Y Combinator. I assume that by writing here, I am putting information out into the world for anyone to use as they wish.
HN/YC cares more about community aesthetics than your right to be forgotten.
Try to have your account and its contents deleted. The best I was offered for my 2011-vintage account was to randomize the username, and the reason I was given was that browsing and old thread with a bunch of deleted comments "looks bad".
AFAICT, you retain the copyrights to your comments, but YC has a license to essentially do whatever they want with them.
So, you could additionally give a license to the world to use your posted comments freely. That doesn't mean HN can't add terms to say clients can't copy the site as a condition for use.