I'm not sure why you're criticizing the article by mentioning nuances as if they didn't also mention them in the article...
The point is that when they average person double clicks an MD file, it should probably open and display.
They're also like a bazillion different PDF and TXT formats, but viewers for them are rather ubiquitous.
You might say that nobody uses PDF viewers because they just opened them in a browser. Well, that's also a suitable approach for MD.
The support doesn't have to be comprehensive. I don't think they're asking for the God of all viewers and editors that supports every different styling and switches between them automatically.
The idea is that if you send someone a markdown file, they should be able to read it without having to go into a menu and opening with a text editor. You know, like Normies do with a bunch of far more complex standards already...
My guess is that this is because Markdown is mainly used by technical folk, and they're fucking stupid when it comes to end-user experience and just haven't considered it much. And the other reason is that the big players would rather push their proprietary formats so they don't want to see adoption of Markdown.
The author is right. Widespread, ubiquitous support is far overdue for such a simple format. Your remarks about cellphones not opening text files isn't a great argument against not opening markdown files; frankly, I think it's absurd that they don't, and that should be pointed out rather than used as an example of why we shouldn't bother for MD. If an OS shipped on a floppy drive could have a text editor out of the box, I'm pretty sure your fancy multi-gigabyte smartphone in $current_year could do it too.
I swear, these comments are depressingly stupid. Imagine arguing about this for any other format...
Yes, markdown should be more accessible and its files should be easier to use. Why? Because everybody uses it already, even if they don't know what it is. That's how ubiquitous it is, and supporting it out of the box is what you're arguing against. It's hard to comprehend. Are you employed by Microsoft?