One of the original ideas behind Markdown was that a document should remain readable even before a computer interprets its formatting.
The author states this towards the beginning of the article. This is possible everywhere you can easily open a txt file. Note, there are quite a few platforms that don't like to make it easily possible to open text files.
Then later, he goes on to say that opening it should interpret the formatting as well. This is neither the original premise of Markdown, nor is it easily doable. Namely, there are a number of different markdown 'specs': Github-flavored, Commonmark, etc. On top of that, Markdown is (mostly) a superset of html syntax. Technically any html is valid markdown formatting.
Thus, in order to 'simply' render the formatting of Markdown, you first need information on what type of markdown, and then also be running a fairly complex parsing+rendering pipeline.
Not to mention that you are doing all that for a document format that the non-tech/average user has never even heard of probably. This is the same reason why syntax highlighting (beyond Regex/Treesitter) isn't part of most text editors.