I dont' think that's true. https://news.ycombinator.com/robots.txt

  User-Agent: * 
  Disallow: /x?
  Disallow: /vote?
  Disallow: /reply?
  Disallow: /submitted?
  Disallow: /submitlink?
  Disallow: /threads?
  Crawl-delay: 30

Links in comments are marked nofollow but submissions are not.