The article mentions affordances. I assume the title uses doorknobs because that's a more familiar word as you point out.

no, "doorknob" is merely higher-frequency due to its other meanings. it's never used in this context - probably because it's a terrible affordance (see Norman - push or pull?)

Doorknobs is a more commonly used word I'll grant you, but it meant noting to me at all whereas affordances would have.