Looking at the site it seems to be a (static?) HTML and shared "main.css" and "main.js" files. Both files can be cached by the browser, so it only needs to download a few KB of compressed HTML for each page. I don't think we would notice much of a difference in the navigation from one page to another if they used content preloading

It's how we used to make websites before SPA, and it's refreshing to see that it still makes a noticeable difference even on today's powerful CPUs and high speed networks.