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.

They are both at the end of the day UTF-8 text.

Isn't markdown a strict superset of html though? As in any valid html is valid markdown. Not usually how it is used though and im not sure about how most markdown flavours handle it, but it is a "convenient" escape hatch

https://www.markdownlang.com/advanced/html.html