Well yeah, I assume they referred to "writing" as in the entire workflow, including drafting, reference, revisions etc.
Because otherwise, I have zero idea what that sentence was supposed to imply. If "serious writing" actually means "typing words in a serious manner" in the original comment, then I don't really have much to say, except that OC is probably spewing utter nonsense.
But nothing about that applies to the writing process itself. You could be like George RR Martin and write in WordStar for DOS and then convert the files over to Word before you send to your editor. The main issue there is how often you need to correspond (which may be seldom for a novelist like Martin).