Exactly - does anyone (well, maybe the author does since he did use markdown in previous works) use "basic" markdown anyway? There are always some extensions (either front-text metadata or syntax extensions like this one) in the "interesting" apps. (The python markdown module API kind of feels like it's entirely about extension and only a little bit about actually parsing, but that did make it easy to add an `ASIN()` token for a review site without needing a preprocessing stage...) That does make it a little worse for interchange, but that's maybe less important for markdown anyway.
(Even with pandoc, https://pandoc.org/MANUAL.html#extensions lists a bunch of syntax for things like this, brought in from github, php, and other places - for the example above, `implicit_figures` and `link_attributes` are a good starting point.)
