First of all, that pose is fairly typical of a formal portrait and is often a mostly frontal shot (usuallly a bit from the side to slim the figure). I suspect what the video is saying it would be a bit odd and unbalanced to chop one arm off in this type of a shot. The rule is totally bogus:
Here is a link to some of Yousef Karsh's portraits. He is considered by many to be the greatest portrait photographer of all time. Notice chopped off arms and missing arms in some of these very well known works..
http://121clicks.com/inspirations/th...y-yousuf-karsh
I think you will find that he has broken just about every rule written about portraiture when you look at these. And they all work....