I mean, you're right... but the goal of an analogy isn't to be perfect, but to be a rough mental model to quickly express a concept. I think it's fine.
Different jobs require different levels of abstraction and design docs fulfil that role for software engineers.
Yes models can be helpful even if they aren’t perfect. But not only is this one too far off to be useful, it does more harm than good. The expectations that happen when people (specifically managers) start thinking of design docs as blueprints, software architects as architects, and developers as builders are downright dangerous.
Different jobs require different levels of abstraction and design docs fulfil that role for software engineers.