A human needs a perfect spec too.
Anything you don't define, is literally undefined behavior the same as in a compiler. The human will do something, and maybe you like it and maybe you don't.
A perfect spec is just another way to dedcribe a formal language, ie any programming language.
If you don't care what you get, then say little and say it ambiguously and pull the slot machine lever.
If you care what you get then you don't necessarily have to say a lot but you have to remove ambiguity, and then what you have is a spec, and if it's rigorous enough, it's a program, regardless what language and syntax is used to express it.
I think the difference is that with a human you can say something ambiguous like "handle error cases" and they are going to put thought into the errors that come up. The LLM will just translate those tokens into if statements that do some validation and check return values after calls. The depth of thought is very different.
But that is just a difference of degree, not of kind.
There is a difference between a human and an ai, and it is more than a difference of degrree, but filling in gaps with something that fits is not very significant. That can be done perfectly mechanistically.