Word automates a lot of things in the editing cycle of a novel. Word's "Track Changes"/"Accept Changes" tools and the workflows they enable are by far the most common in a number of industries, but especially in edit workflows of most major publishers. That's why the Word format specifically is often requested/required, because people know and understand those tools and already have processes/workflows built around them.
The closest equivalent tools for plain text are source control systems and text diff/patch. Those tools are great (and arguably fit some definitions of "superior", especially in capturing history earlier in the process and keeping history longer after the process). They are also nowhere as ubiquitously installed as Word, and nowhere near as easily taught (or already known/understood) as Word in today's world.
Those "edit cycle" needs maybe aren't a huge reason to do the initial writing in Word, but they are certainly many steps above "no value", and thus the noted restriction that no matter what tool you use for the initial writing, if it doesn't have an easy Word export to get it started down the edit cycle path then it doesn't have an easy fit in today's publishing world.
The point of using markdown is that you got many tools which are superior is aiding in you writing process and organizing your text.
The final format is much less important than whatever aids your writing process.
I have tried using traditional word processors for writing novels and I cannot say they measure up to tools like Ulysses.