The site was assembled using webpack and at least one element has a `data-react-helmet` attribute, so my guess is it's a React or React derivative such as Next.js. You're right -- it's not progressive enhancement and it's not necessary to present a simple tutorial.
Maybe there are other reasons, though? Maybe Ben wanted to learn the framework? Maybe Ben wanted to integrate a storefront and a comment section -- which he has done via Shopify and Disqus -- and his choice of framework made that easy due to the existence of official React libraries from both vendors? Maybe Ben's using cloud-native serverless deployment tools alongside his React application because some of the derivative frameworks like Next.js do really well at minifying, compressing, caching, and serving only the content that's needed?
I can understand the purist argument -- I really can, especially from a security standpoint given what we've learned in the past few years about cache-timing side-channel attacks -- but is it possible that Ben is simply a fan of learning and he's as excited about building a web application in React as he is about building an 8-bit computer from scratch?