I mean, Markdown was specifically designed to be convertable to HTML. So, it is structurally rather similar. It's basically just a bunch of aliases for a subset of HTML tags.

Having said that, it is also designed to be plain text. You don't have to convert it, and even when you do convert it, there's a good chance that what you've written in plain text will look fine in converted Markdown as well.
For the purposes of this meme, I would certainly group it with plain text.