I did not know a lot of the history outlined in this post by Caroline Jarrett until she kindly pointed me to it last year. Historical drama aside, I think the key distinction for me is the importance of testing content with users.

Plain language relies on testing with users. If the intended users can use the content to do what they need to do, it’s plain. If they can’t, it’s not plain. So you can only really know whether you have succeeded in writing in plain language when you have tested with the actual users.

And of course, this detail is fundamental to content design as a discipline. It's about following the principles of clear writing, but making decisions based on research and data. You need to understand what users need before you start writing. Then you need to test your content when you're done to make sure it meets those needs. It's only plain or clear if it does.