The author explain this in more detail in the article; I also was initially confused. The key details come from their broken down pandoc command. Specifically, these two flags:

  --pdf-engine=typst
  -V template=article.typ

Despite the input content being a .md file, we see that the .md contents populate the .typ file. Pandoc then understands how to convert the populated .typ file into a .pdf. The author also notes in their hyperlinked .typ document where the body content is placed:

  // THIS IS THE ACTUAL BODY:

What is not clear to me is the actual mechanism that tells pandoc where to place the body contents in the template. I presume it's some "magic" from the pandoc docs. The pandoc templates documentation[1] does reveal to us that `-V` is setting a variable called `template`. I don't have pandoc locally, but from another post[2] from the same author and a chatgpt query, it seems that the `body` symbol in the template is the chosen substitution symbol.

[1]: https://pandoc.org/demo/example33/6-templates.html

[2]: https://imaginarytext.ca/posts/2024/pandoc-typst-tutorial/