The problem is because you're using GitHub Pages to host your website on a custom domain. When someone accesses your website via HTTPS, they get a certificate that is only valid for GitHub domains. Of course the HN submission is an HTTP link, so the person you're replying to probably has their browser configured to use HTTPS everywhere.

You can fix this by enabling HTTPS in GitHub pages, so that a valid certificate is served. See GitHub's troubleshooting information: https://help.github.com/en/github/working-with-github-pages/...