But that doesn’t make much sense - by your account Latex would also be a mix of closed and open source, since closed source web apps exists for writing Latex.

What does not make sense? Did you mean to answer to someone else? I only stated that Typst (the typesetting engine) is free to use and modify, and only the web app is closed source. Typst can be used without touching any web apps. I use Typst locally.

I made no claims about any mixes or claims about LaTeX.