Yes. And I had read it slowly twice before my last post, but unfortunately, the more I read, the more confusing it got as regard to what is “an external document”.
(I think that the fact that Scrivener has a function to “copy document as external link” only made it worse for me. Even more so that these two topics are consecutive to one another. 10.1.5 and 10.1.6 [See ※ at the bottom of this post])
[Keep in mind though that I initially thought that including a document from another project at compile was in fact a possible/intended use of the <$include> placeholder, and that for some reason it just didn’t work at the moment. → In other words: I started biased.]
For example:
that sentence (which is without italics in the documentation) could (and in my opinion should) be improved.
Because, as it is at the moment, it can be read both ways:
To insert the main text of → [any document in the current project]
(which I now understand as being the intended signification),
but also:
To insert [the main text of any document] in the current project
(to insert → in the current project → the main text of any document).
And “document” being what is internally to a project a file named throughout the manual, that can easily get someone on the wrong track in my opinion. (It sure confused me.)
I’d personally reformulate it: To insert the main text of any document from the current project, […].
Although not perfect, since that leaves out the fact that it is possible to link to a “freestanding” file. (As named it MM.)
Anyways, so all in all, what in my previous post I described to be the way I managed to make it work, is actually the very way the feature is in the end intended to. (?)
Although it still leaves the question as to why then, since a document in a project is already an RTF file on disk (and unconditionally up to date, to top it off), one couldn’t simply link <$include> to it.
Wouldn’t that be an answer to any user asking for or about “library” functions/features ?
※ Right off the bat, section 10.1.6 starts with:

At least now it is clear to me.
Not a big deal, I was only curious about why it wouldn’t work for @Tobias_technicalauth to begin with, and decided to give him a hand.
(At which, if I succeeded, was by failing. lol)
And thanks for the replies. 