It may not significantly affect users of modern browsers in their default configuration with good internet connections, but it does affect plenty of other things.
You want to parse things in the document? Now you need a whole different suite of tools from the usual tools you use. Your library that parses all the meta tags, identifies the content, &c. is now useless. Now you need either a full JavaScript execution environment, or a JSON parser instead of an HTML parser (and that JSON has completely lost the semantics that HTML provides, so you can’t query things like “document title” or “meta description”).
You have JavaScript disabled? Here, have a mess that, well, it’s better than most client-side rendering things in that the content is still probably there, rather than the screen just being blank, but there are reasons why you should always prefer server-side rendering for things like blogs.
You have a slow or unreliable internet connection? Now the page is taking longer to load, and until the JavaScript loads, the page is empty—and it may fail to load.
I object to people doing things like this as more than a fun technical demonstration because it does harm some users.
They're no less useless than for SPA's that render their content in JS. That's all this site is. If your web scraping suite can't handle content loaded from JS then you're already locked out of most of the web.
Same if you disable JS. Most of the web will be broken for you and it takes an annoying amount of effort enable the minimum necessary scripts.
The key thing here is that SPAs normally get some kind of interactivity benefits from being written in that style (though I confess they break things that the platform provides, by reimplementing them badly, at least as often), such as loading same-site links faster. But this thing doesn’t do that; it’s purely a projection, like XSLT. It should be done as part of a generator or server, rather than on the client side.
I can imagine the author had their fun, I also had fun reading the article, but I don't think this is leading to an accessible internet.
Interesting idea, but Google has a dominant position in both the browser space and the search-engine space. They can punish such pages in the Google search rankings, and this avoids the obvious retort of Why are you deliberately making my browser worse?