It depends on the context. What you say makes a lot of sense for web sites, but expecting a web app to use JS strictly as a "progressive enhancement" is IMO unreasonable.
Now, this particular website is indeed a proper website, and so it shouldn't need JS to do its thing. But it's also a website that advocates for a certain way of developing web apps (even if they don't use such terminology themselves), and as such, is essentially a demo for the same.
This is quite fair point, yet I'd argue that still most so called web apps could (and should) use basic old-school HTML forms as the underlying technology, and progressively enhance from that baseline, up to the "app-y" look and feel we know and ~~ha~~__love__.
Obviously, there are some limits where application built with bog standard HTML forms becomes too cumbersome or makes no practical sense, as you say, but I think that that threshold is far higher than what current web-app landscape exhibits. I think the threshold is around video editing software, or real-time multi-user collaborative spaces perhaps. But for the rest, following three old steps
Make it work — just HTML.
Make it nice — add some CSS.
Make UX slick — add JS.
still makes sense to me. In context of the plainvanillaweb.com it would mean just moving the content from non-semantic attributes of custom components to their semantic initial content, such as adapt
Now, this particular website is indeed a proper website, and so it shouldn't need JS to do its thing. But it's also a website that advocates for a certain way of developing web apps (even if they don't use such terminology themselves), and as such, is essentially a demo for the same.