At the time "generating a program from a spec" was an idea floating around that you could come up with a "spec language" that was easier than regular programming languages but somehow still had the same power and could be compiled directly into a program. That's the crackpot idea that Joel is referencing - but that's not what a spec language used with an LLM is doing.
[1]: https://www.joelonsoftware.com/2000/10/02/painless-functiona...