If you look at it strictly from a jujutsu aspect, if the arm is straight, then it can be used as a lever to affect someone's hips. Get the hips to be out of alignment with the rest of the body and nikkyo is easy to apply.
Looking at it from an aiki perspective, the lever isn't going to work. The hips won't be affected because of how the internal body structure is built.
The unbendable arm trick is only one half of what really should be going on. While it gives some strength when you mentally extend through your arm, it isn't the complete exercise. Intent going outwards should be combined with intent coming inwards at the same time. Not an easy thing to do or keep going in a dynamic situation.
the problem with this stuff is that you really need to feel it. The folks who can look at the video and see what is happening are already working on it and the folks who aren't, largely can't see it. Allen's point is spot on.