The point in the video that you are missing is that the material when new, has oils that makes it spongy and more effective. Those oils evaporate with time. The demonstration with the hammer is just to show very casually how brittle the material becomes compared with a new one, and the difference is evident.
The study you linked, as yourself said, is for bicycle helmets. They are not designed to protect you against the same amount of force as a motorcycle helmet.
edit: typo
They are not designed to protect you against the same amount of force as a motorcycle helmet. That study just proves that expired bicycle helmets are still good for bicycle accidents, not motorcycle accidents. I rest my case.