Before AI era, specs meant documentations. The specs of jpeg, png, h.264, docx, usbc or whatever format you can think of, are not just a bunch of C++ source files.
Compilable source files served as example implementations, complementary to the specs themselves. The idea that code is spec is just agile brainrot, like the idea that clean code means you never need to write documents or comments.
Edit: it's very interesting that all the other engineering principles know the difference between spec and implementation, except in software engineering the idea that all you need is code got reinvented every few years. Every generation thinks they're the special one, I guess.
Compilable source files served as example implementations, complementary to the specs themselves. The idea that code is spec is just agile brainrot, like the idea that clean code means you never need to write documents or comments.