[therapeutic]

I think the first comment in that thread is what you're referring to. I'd hazard a guess that a "fullstack website" in this context means a site that is written entirely in Javascript (ex. React for the frontend and Node.js for the backend). The original thread question was looking for ways to improve knowledge in Javascript, so building a site entirely in that language would certainly be sufficient to give one an understanding of what you can do with it,

[tylerhou]

Since the commenter said "full-stack website with JS" I would assume that the commenter intended "full-stack" to mean both backend and frontend. If he meant "full-stack" to mean entirely Javascript there would be no reason to tack on "with JS".