It's kind of a gray area, but headings are scoped to sectioning elements so there can be multiple h1 in a page. After all, article content should be considered as stand-alone sections of content. I'm not aware of how this affect accessibility.
But at the same time, I get that this guide seems aimed at people not very experienced with HTML so it may be fitting to deal with details like that later.
> Avoid skipping heading levels: always start from <h1>, next use <h2>, and so on.
Isn't quite correct. For longer documents it is sometimes appropriate to skip a level if it is warranted. For example, an online book may have an <h1> for a chapter heading, then some text introducing the chapter, then a minor point that doesn't deserve as much prominence as a proceeding section. So under a chapter called "Introducing Python" (an <h1>) there may be a heading immediately following the <p> text after the <h1> called "Python Runtimes" that isn't really an important part of the document, but deserves to be covered before any <h2> is called. It's ok to label it <h3> so that it shows up in the table of contents appropriately as a minor section.
It also makes more semantic sense and is better for screen readers too. If "Python Runtimes" were a <h2> a reader / lister could reasonably assume that this is a semi-major part of "Introducing Python" when it is not. It's just a quick interlude that says "There are many types of python, you should probably use CPython, since that is what this book is going to assume" or something like that.
But like you say, this is geared towards junior devs, so it's ok to generalize a bit.
It's a new addition, so I'm giving it time to see how it's going to be used in the wild and although I kinda like OP's take, the ones with giant images, GIFs, too much text... I'm not so sure. Maybe it's just like this now because it's a shiny new toy and people are pushing its boundaries but will die down later.
I'm lightly skewing towards just hiding it with a custom rule and forget about them. I visit a Github profile to quickly see what the developer codes instead of a showcase of their personality which, as you said, would be better served by a personal website.
But at the same time, I get that this guide seems aimed at people not very experienced with HTML so it may be fitting to deal with details like that later.