From The Medium wall, Don Hopkins on how he builds those posts:

I think of what I try to do as manually implementing Ted Nelson's, Ivan Sutherland's, Douglas Engelbart's, and Ben Shneiderman's important ideas about "transclusion".

Following links is distracting and costly, so most people aren't going to click on a bunch of inline links, read something, then come back... So I aspire to simulate Ted Nelson's and other people's ideals with the crude stone knives and bearskins that we're stuck with today.

The irony he names: he had to transclude by hand in order to explain transclusion, which Ted thought should be built in and automatic.

This reader takes a transclude fence naming an article — and optionally another database — and splices that article's current segments. Change the source, the illustration changes.

The Founders in this database is a door: its body is a transclude of the 1988 demo. Sun Founders Big Heads transcludes that door. Nested, live, not copied.

A span still needs a start and an end. Right now a transclude is a whole article. That is the next cut.