There's a misconception that a writer with a Masters or PhD will be the best writer.
Agreed. Most PhDs have only written one dissertation in their entire careers; and more often than not, they spent much of that writing time stumbling around just trying to understand how to do it and/or making extensive revisions necessitated by the fact that it was the first (and only) dissertation they ever had to write. An experienced writer rarely (if ever) has the same substantive academic background as a PhD candidate, but writing a dissertation doesn't require the same substantive understanding of the entire field as that possessed (one would hope) by any PhD candidate.
First, a dissertation covers only a very small slice of the broad scope of a PhD program rather than requiring a broad comprehensive understanding of the whole field. Second, dissertation writing is an "open-book" process, not a test of what knowledge the author of that dissertation has. Third, any highly-experienced academic writer knows how to research a topic and write about it relying on authoritative material at the appropriate level better than just about
anybody who hasn't done this for a living for a decade or two. Finally, most dissertation candidates will be able to provide or direct a writer to all of the same sources of information upon which the candidate would have had to rely if the candidate had undertaken to write the dissertation without the help of a professional writer. Once the writer has access to all of those sources, the principle remaining variables become simply: 1. raw writing ability, and 2. experience writing dissertations.
Certainly, the first time I ever took on a dissertation, it was, undoubtedly, a very intimidating project. It came out great, but it took me much longer than similar projects take me now, after having produced so many of them in the last 10 years, and involved a lot more stress. While I wouldn't presume to be able to discuss random topics in any academic field as well as a PhD-holder in that field, (at least not spontaneously), I'm quite confident that I can now write a much better dissertation in virtually any field that I handle than the vast majority of PhD candidates in that field, especially if I have access to all of the same authoritative sources.