It's a bunch of static HTML files, because why do you need more? It's a company website. It's run by one person. Anything more complicated than this is a waste of CPU cycles.
People need to just write HTML and CSS more. Keep it simple. Don't overthink this. Maybe some Javascript, too. This site doesn't use anything other than what's available to a browser. It doesn't use jQuery. It does use Google Fonts, but that's because why not.
I mean, come on. It's a website.