Joke aside, the source code isn't featured on the web page, despite LibreOffice (unlike Apache OpenOffice) being copylefted. They mention self hosting, but it says to contact them. It seems it runs on WebAssembly either directly or through a VM on top of WebAssembly like https://copy.sh/v86/ . The screen seems to be simulated with a canvas. I think perhaps they didn't change the code of LibreOffice much, but have a custom setup of something like https://copy.sh/v86/ or even are running it on actual WebAssembly with a virtual screen. So they have a virtual computer that it's running on, and they consider that separate when it comes to copyleft obligations. Plus it's possible to make something open source but inconvenient, and charge to make it convenient.