Not the author, but presumably you're overlooking the fact that the expounded term doesn't necessarily have to be "inside" or even "neighbouring" to the details element.
The author's intent here is to have terms explained in the text explicitly in such a way that it would 'augment' the text with an explanation somewhere further down the line, but not necessarily "in-place".
It is also intended for text specifically, rather than replacing one element with another.
I agree that display/summary are similar in spirit though, I had not come across those before.