I guess it depends on how much you care about archery, which you lot don't seem to (Ryllharu didn't critique the article's critique of JR's form), then you might look harder at the form and technique of "the world's greatest archer", even if it's in a movie. Like I mentioned, it makes me laugh to see police and special forces characters on tv/movies who can't hold a firearm correctly. It ruins the fantasy, if ever so minutely.
The author claims to be an instructor so I think he's basing the arm guards on JR's form. Seeing that poor form, he expects the archer takes a lot of string hits to the forearm, hence he concludes the need for 2 guards. It may very well be a prop/part of the costume. I imagine if the pain was so bad from repeated strikes, then JR would have worn a larger forearm guard instead of just doubling up.