Why would it be in the public domain? Anything you create, under US copyright law, is the opposite of being in the public domain, it's yours. According to the legalese of YC, you are granting YC and YC alone a license to use the UGC you submitted to their website, but if anything, the YC agreement DEMANDS that you own the copyright to the comment you are posting.
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> You acknowledge and agree that any questions, comments, suggestions, ideas, feedback or other information about the Site (“Submissions”) provided by you to Y Combinator are non-confidential and Y Combinator will be entitled to the unrestricted use and dissemination of these Submissions for any purpose, without acknowledgment or compensation to you.
Another example of this is people putting code, intended to be shared, up on e.g. Github without a licence.
Many people seem to think that no licence = public domain, but it's still under strong copyright protection. This is the point of things like the Unlicense license.
> User Content Transmitted Through the Site: With respect to the content or other materials you upload through the Site or share with other users or recipients (collectively, “User Content”), you represent and warrant that you own all right, title and interest in and to such User Content, including, without limitation, all copyrights and rights of publicity contained therein. By uploading any User Content you hereby grant and will grant Y Combinator and its affiliated companies a nonexclusive, worldwide, royalty free, fully paid up, transferable, sublicensable, perpetual, irrevocable license to copy, display, upload, perform, distribute, store, modify and otherwise use your User Content for any Y Combinator-related purpose in any form, medium or technology now known or later developed. However, please review the Privacy Policy located here for more information on how we treat information included in applications submitted to us.
> You acknowledge and agree that any questions, comments, suggestions, ideas, feedback or other information about the Site (“Submissions”) provided by you to Y Combinator are non-confidential and Y Combinator will be entitled to the unrestricted use and dissemination of these Submissions for any purpose, without acknowledgment or compensation to you.