I think they were using "design" as akin to the blueprint to the building. I totally agree with them that in that sense, the code is the blueprint, not the building. The execution of the program is the analogy to the actual construction: the builder (computer) takes the blueprint (code) and builds the building (executes the program) based on it. The power of computers is that the "building" (execution) part is essentially free and infinitely reproducible. But the creation of the "blueprint" (code) is still labor intensive.
The right analogy for design artifacts really is to the higher-level vision definition for a project. Engineers don't just show up and start drawing blueprints; one or more people come up with a purpose and concept for a building project, brainstorm approaches, evaluate trade-offs, come to some consensus on the right direction, and only then start working with engineers to start creating and iterating on blueprints. This vision phase is where design docs fit.
The right analogy for design artifacts really is to the higher-level vision definition for a project. Engineers don't just show up and start drawing blueprints; one or more people come up with a purpose and concept for a building project, brainstorm approaches, evaluate trade-offs, come to some consensus on the right direction, and only then start working with engineers to start creating and iterating on blueprints. This vision phase is where design docs fit.