Markdown is a lot less verbose in my opinion than RST. It's not intentional, nor is it insurmountable, but little things like inline code, hyperlinks, defined subsection levels, and many more all add up to a bit of cognitive load that I don't need for all situations. When I'm writing quick notes for a meeting, or trying to jot down something, RST is a bit much. Which perhaps isn't what this article is really harping on.
But when my quick notes to myself about a project turn long-winded and become more of functional docs, it can be a pain to rewrite that into rst.
I do, however, tend to use RST when I sit down to write documentation for a project. I even tend to use it when documenting firmware and hardware portions of projects. This of course due to the so far poor handling of images in Markdown.
So like I said above, they are tools. One may be better suited to something than the other. But thus is the nature of tools. Use the right tool for the right job whenever possible.