That's true! Let's Encrypt doesn't provide a web-based service to obtain certificates, only an API that software can use to request certificates. Because of this design, there are dozens of different tools that people use to get certificates from Let's Encrypt:
cPanel itself includes one, but it can only be used if the hosting provider permits it.
The web-based clients that I mentioned are third-party tools that use Let's Encrypt as the back-end certificate provider.
(Certbot, which I've worked on, is a program to obtain certificates that's mainly aimed at the case where you have shell access and preferably root access on the server—commonly on a VPS or dedicated server, rather than shared hosting.)