It can fall under any of these with minimal changes, so your question is not very meaningful. I'd say it is better suited for the application web though, because you expect interactivity. The point here is that there are not many documents that do necessarily need interactivity.
But that article is exactly what we want on the internet. It's the kind of interactive teaching demonstrations that forward looking educators have been thinking about for decades. It's not bloated or overdone; it doesn't take up significantly more processing power; it works far better than a traditional article ever could. This is what we desire the internet to be like.
It's because of articles like this one, or 3Blue1Brown videos, that I feel like the idea of "the document web" is really a step backwards: the internet is not just a glorified transmission method for paper anymore. We have computers, which are able to do so much more than paper ever could. To not exploit them for their capabilities, simply due to fear of exploitation, is far worse. How much worse do you think https://thebookofshaders.com/ would be to learn from if it couldn't include live editable demnostrations? To divide up the web into "interactive" and "non-interactive" content is to remove the ability for creators such as the author of the Mechanical Watch article to add progressive interactivity to their documents, remove the ability for learning resources to fully utilize the power of computers, to remove the ability for small fun things to be added to sites such as a Konami Code easter egg or https://bruno-simon.com/'s interactive website. And before you reply with something like "these would all go on the interactive web", my point is that dividing the web like this would not only be pointless but would also be ultimately harmful to the creation of things such as these.
Don't get me wrong, I like interactive documents a lot, but they also take a lot of time and effort to produce and thus will remain a minority. Texts greatly outweigh every other medium primarily because they are very easy to produce and distribute.
> they also take a lot of time and effort to produce
This is a case of bad tooling, not an intrinsic fact.
https://www.joshwcomeau.com/css/understanding-layout-algorit... is an article that could have easily been written without any interactive components. All the author did was replace traditional code blocks with an embed to a web playground (e.g. by s/Code/Playground/g in a MDX file). Ta-da, the demonstrations are immediately interactive.
> and thus will remain a minority.
Not if we put in effort to encourage them. On the other hand, if we put in effort to _discourage_ them (say, by splitting the web into documents without interaction and apps with interaction), they will disappear completely. It is not worth sacrificing these creations simply so that we can remove a few bad actors. (Mind you, there are many ways to act badly in non-interactive ways too. Image watermarks, embedded advertisements in videos, embedded advertisements in ASCII text...)
> All the author did was replace traditional code blocks with an embed to a web playground (e.g. by s/Code/Playground/g in a MDX file).
Well, not surprising given that the article was about the web technology (which powers the application web, of course!). I might have been convinced otherwise.
There is indeed some negative feedback loop going here. But I think the cost to achieve interactivity has been decreasing over the time (compare with, say, 1990s' interactive CD) and the gap is still very significant, even with a very favorable condition (nowadays you can practically have interactivity anywhere anytime!). I'm happy to be proven wrong, but for now I'm not optimistic about that.
