What makes folks thing landing pages like this should require JavaScript? Nothing here would need the dynamic capabilities of a scripting language, yet accessibility & SEO are hurt in the process.
You can read the 20 lines of JS that they use. It's for drawing the parallax lines in the background and for the fade-in effect, which can't be accomplished with plain CSS. There's no detriment to SEO or to accessibility, unless the specific disability you have is that you're allergic to JavaScript.
There's an old adage about hammers and nails... I forget the details.