This is what I though as well when I first looked at their website. But nope, the compiler and LSP are available as fully offline programs under the apache license. But I understand how you’d get confused, their website is strikingly polished for an open-source non-commercial project!
The only thing you’re missing out on if you use the offline version is having the rendered preview update in real time as you type, but you can sort of emulate that feature using their neovim plugin and a really fast PDF viewer like zathura.