How is this news? React SPAs have been around for years, and SPAs existed long before React too.
Though almost no one is able to do it.
Which is weird, as it's objectively easier than writing enterprise react crap.
The example URL is a complete website within a single HTML document with no external dependencies and no further round-trip requests. It is a single-page site (SPS) or perhaps a multi-page file (MPF).
https://john-doe.neocities.org/
You can open that URL, disable networking, and browse the entire site to your heart's content in any browser supporting CSS.
If you open the file in a terminal browser (lynx, w3m, elinks[2], etc.), you'll see the full site presented at once, as a single page, without needing to specifically navigate between them (you can scroll the full site). Though the intra-site navigation itself still works --- it just doesn't reveal or hide sections.
BTW, the discussed page is not at all a single-page site; it makes separate network requests for CSS and PNG files.
The concept of an "SPA" refers to the appearance rather than provisioning of the app, and inherently relies on Javascript (or an equivalent scripting capability) to interactively rewrite the display. It's possible to single-file an SPA. The characteristic isn't central to the SPA concept, and in practice implementation is typically anything but.
SPAs are not accessible without Javascript, and don't render at all from a terminnal / console-mode browser. (Ask me how I know this...)
This is not an app. It's a website, or at least, multiple web pages, provisioned from a single HTML file.
Yes, this instance has several external references. I've noted CSS in an earlier comment, you mention image assets. The concept could be further optimised for portability by incorporating those inline.
Note that optimisations are also trade-offs. Inlined assets would mean duplication for a larger site. Which of those trade-offs are preferable or unwanted really depends on the specific goals.
But as a demonstration of an idea, this really is pretty elegant.
There is a concept of a single file app (SFA). You can share an html file, and that is the application.
It can have inlined micro js/css frameworks, or hand-rolled everything. An SFA should be human readable though- viewing the page source is useful.
I think it’s a concept worth exploring.
No build tools required. Take this file and open it with a web browser. Then modify it, and refresh the browser.
Distribution and Development for applications is about as straightforward and accessible as it gets.