You're of course right, that page is serving HTML/CSS/JS - but that's kinda nitpicking.
It roughly akin to me saying "my Golang on Linux program is 100% C code free - no C is used in the execution of my code!" .. well, clearly that statement would be false.
However, it's perfectly legitimate to say my project is C code free - and that's what this project is saying. It's saying the project is web-tech free. There just happens to be an off-the-shelf execution environment which uses web-tech.
Perhaps if you don't mind fuzzy hand-wavy claims that are as likely to mislead as inform, they could say "our project's source code includes no 'web-tech'.
But that's not the headline. The headline was instead that it's a service "without a website". But there is absolutely, positively, no-nitpicking-involved an HTTPS-accessible website for the service, set up by the service's own creators.
It roughly akin to me saying "my Golang on Linux program is 100% C code free - no C is used in the execution of my code!" .. well, clearly that statement would be false.
However, it's perfectly legitimate to say my project is C code free - and that's what this project is saying. It's saying the project is web-tech free. There just happens to be an off-the-shelf execution environment which uses web-tech.