The application != the landing page.

The landing page is to attract and interest new users who are not yet using the application.

Therefore the landing page should be accessible to the widest variety of users, with the widest variety of setups.

The landing page can also indicate somewhere that the app itself requires javascript, such that users are warned they need to turn javascript on if they choose to use the application.

But the landing page should never, itself, require javascript to be readable/useful as an advertisement for the application.