The source code always was the result of that distillation, or at least a crucial part of the result. I’m unconvinced that natural-language specs can replace that role. Even long before LLMs, when writing specs I often resort to some form of formal language to precisely nail down certain parts of the specification. And some parts are always left open, where I know I’ll get to them when writing the code, because they are more effectively solved “in the code” than waterfall-like up front.