I don't know about the Tomiki thing, but the 'unbendable arm' exercise you describe is easy for beginners because it is a trick.
If by trick you mean that it is learning to use your body in a situation where it interacts with other peoples bodies directed by their intention. Yeah it's a trick. Nobody says its magic. Next you'll be asking me if I weigh the same while performing the 'weight underside trick' as I do when I'm not doing it. Of course I way the same. Nobody is suggesting that this violates the laws of physics, only that this stuff enables you to learn and visualize the proper use of your own body for correctly applying aikido techniques. If you want to tie wieghts to arms and have levels and pullies etc.. well duh. Of course my arm will bend, this was never the point of the exercise in the first place though.
Despite this I wouldn't dismiss it as a trick. It is a valuable learning tool. "No unbendable arm. No aikido"