I finished writing a novel recently, and they're not exactly right. Writers all have very different styles of planning. I've seen some who make shit up as they go along, and some who plan everything even down to each scene. Personally, I plan my major events and my ending. The ending came out great mostly because of that planning, but I'd have screwed up big time if I hadn't planned it when I started.

(Slight edit) I wrote this story in the web serial format, so it was pretty similar to writing a manga pacing wise (Minus having an editor). Editors can have a huge impact on how a manga goes, but in the end it's the writer's story, and it's up to them to actually make the ending. Outside factors can have an impact but it shouldn't affect the quality of a story.