[wetmore]

The text is still readable as plaintext, but I think you could concede that styling it a bit does make it easier to see the effect of your markdown.

It's like how, say, Sublime text has a syntax highlighter for markdown files. Sure, you could argue that is enough to signify that the contained text is bold, but it's hard to deny that also bolding that text in the editing stage is helpful.

Syntax "highlighting" like in OP's editor will always provide more than just plain text formatting can. So it's not really antithetical anyway. Sure, markdown is made to make written text as readable as possible in plain text, but that doesn't mean it can't be even more readable in a non-plain text setting.