You're not wrong per say, but that's not really what this guide is about. It's more about self hosting open source applications and services, not simple web pages. So I don't see how Just make an index.html and put it where your web server can serve it makes much sense here.
As someone who enjoys self hosting my own applications like Nextcloud, Ghost, Matrix/ Element and others, I think using Docker containers does actually makes sense here. I'm also a big fan of the KISS approach, as you are saying. However if you are trying to orchestrate multiple apps on a small home server or SBC, the installation and maintenance can become a huge headache very quickly. Especially if some of those apps have conflicting dependencies.
I'd still disagree. Want to share an image? Put it in ~/www/. Want to share a video? Put it in ~/www. Want to share some music? Put the .mp3 in ~/www. This core idea, that you can share files simply by using a file system, makes many of these extremely complex tools that require containerization due to their rapidly changing deps... overkill. An impediment to the actual goal of just showing people things. There's almost nothing simpler than using your mouse to drag a file into a folder or $ cp etc etc.
This entire thread reminds me of the inverse relationship between actually blogging and messing around with blogging setups, https://rakhim.org/honestly-undefined/19/
As for hosting your own Matrix homeserver, sure, that's such a heavy and over-complex protocol that changes rapidly and uses libs that change even faster. You'd want to containizer that.... and probably give it it's own computer instead of your desktop or an SBC since it's going to take a lot of system resources. Or even better, don't use protocols that are so unstable you can't run them natively.
As someone who enjoys self hosting my own applications like Nextcloud, Ghost, Matrix/ Element and others, I think using Docker containers does actually makes sense here. I'm also a big fan of the KISS approach, as you are saying. However if you are trying to orchestrate multiple apps on a small home server or SBC, the installation and maintenance can become a huge headache very quickly. Especially if some of those apps have conflicting dependencies.