As someone who grew up around medival european castles, the reason is that righthanders with weapons going upstairs will have a harder time using their weapons than righthanders defending the stairs downstairs.
When you are inside such a narrow stairway this fact becomes very obvious.
No, the article is arguing that nobody knows for sure what the reason is. What was suggested could very well still be the reason, just nobody knows "for sure".
Either is was very obvious to the authors (so not included), or not thought to be an important factor.