they used to use a self-signed certificate, but no longer!
they wish to thank letsencrypt.org and all their sponsors for providing a global method to secure sites with SSL for free, globally.
they encourage you to make use of their tools, and to do what you can to make their lives easier, whether donating money, time, intellectual property, bandwidth, servers, or merely verbal support such as this.
Oh, and they wish to also note that you CAN still use a private CA’s client certificates to authenticate against a server protected with a letsencrypt.org certificate!