Thanks, yes, that is indeed what I do—I do have DT set to open markdown files in iA Writer when I double-click them, like you do with Pages, Numbers (I also do that as well).
When I say “multi-step process”, I am talking about the whole experience of working with multiple text files and reference documents stored in DT while writing:
Opening a markdown file from DT into iA Writer is one step, yes. But now I have two apps that I’m working in; typically I have reference documents from DT open in DT that I want to look at while I write in iA Writer. So then I’m switching back and forth between apps or moving windows around from different apps so I can see them at the same time. Even if I’m not doing that, but just making a quick note in a single markdown file, still, when I save and close the file in iA Writer, then I have to do an extra step to get back to DT. That’s multi-step. It may not sound like much but when you’re working across dozens of text and reference files on various different projects throughout the day that’s a ton of extra back and forth and clicks. Life would be much simpler for me if DT had a markdown editor that looked more like iA Writer’s. The OP’s screen shots above comparing the look of the markdown editors illustrate well the issue for me.