It is a bit funny that young people seem so far removed from handwriting nowadays, that they miss the obvious. Either that, or Martin is ambidextrous.
Though, I think the author's point still stands here, this could have been done a different way and is largely customary. I am left-handed, for example, and I write up/down lines as parallel to the line of my arm in order to copy the rightward lean, and this also feels perfectly natural -- if right-handed writers had chosen this instead originally, we may have a left-lean on our letters.