I don't think it does at all!

> The overriding design goal for Markdown’s formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions.

https://daringfireball.net/projects/markdown/

Using some semantic HTML as an occasional escape hatch is perfectly in line with this overall goal.

That doesn't make it not true though. Markdown generally supports HTML (though oftentimes only a subset), and is typically styled using CSS. Using a web view makes complete sense to me.