I couldn't find any references to copyright assignment in the HN Guidelines or FAQ, so I'm assuming that users of HN retain the copyrights to their own comments, and except for cases that fall under "fair use"[1] (e.g., criticism, educational use, etc.), they can't be reproduced without the author's permission.
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[ 4.3 ms ] story [ 11.7 ms ] thread[1] https://en.wikipedia.org/wiki/Fair_use