I don't think the un4seen site is actually "supposed" to support HTTPS since the main site offers a self-signed certificate (but of course it would be nice if HTTPS was available there, e.g. through Let's Encrypt). The error on the support site (which afaik is not run by Ian, so he probably cannot do anything about that) indicates that Windows XP probably lacks the required root certificate. Chrome uses Windows XP's root certificate directory, so you either have to install the correct root certificate, or upgrade to an actually supported operating system, or switch to Firefox which brings its own, up-to-date list of trusted root certificate list.
Either way, Let's Encrypt's announcement to support Windows XP has nothing to do with any of that, because in their case their certificates contained a directive which was not understood by Windows XP's certificate validation procedure, but that has nothing to do with untrusted root certificates.