I expected this to be implemented with something like document.body.textContent = "Please disable JavaScript to view this site" on page load. Which would be enough to work. It's actually exactly it, but with the extra precaution of wrapping the entire page inside a noscript tag. Hilarious. I guess it's useful to avoid having a chance to see the content during a flash, before the script executes.
A good contrast with web pages which are not apps telling you "This app requires Javascript to run".