Obsidian does nothing of the sort. That is standard MD.
Obsidian (and others) use [[MyPage]] to create a link to MyPage (as does DT). Obsidian devs decided to extend that syntax to say if there is a ! prepended then display the item vs. create a link, borrowing from the MD syntax.
Just like [MyImage](http://myserver/MyInage.jpg) would create a link to image and ![MyImage](http://myserver/MyInage.jpg) displays it.
There isnt a standard transclusion syntax for Markdown. There is a standard image syntax which Obsidian does support because it uses CommonMark. But Obsidian lets you transclude multiple file types like images, PDFs, etc.
I believe @jasonekratz is correct: there is no true standard for Markdown syntax, and MultiMarkdown is a dialect or derivative (and mostly just one person’s idea of a direction to take). Efforts to standardize Markdown have been drawn out and controversial, and Gruber seems to dislike the current efforts (c.f. Standard Markdown Becomes Common Markdown then CommonMark).
Regardless of Gruber’s feelings on it, he essentially defined a loose standard the moment he defined his schema.
And MultiMarkdown has often been seen as a natural extension of the original spec, while honoring the core he defined.