I think something can be considered a spec to some degree if outcome was derived from it.

It might underspecify, but that is just a quality of the spec. And as long as there is any discretion at implementation time, then the spec necessarily underspecifies to some degree. The goal of a spec is often to be clear on key directional concerns. Consider RFCs.

There's a trade-off between specification and implementation discretion. But if you think about starting at one end ("an internet forum") and then incrementally expanding the specifications ("it has topics with many posts in a linear paginated list"), there's some point where everything you care about is in the spec, and the rest is left to implementor discretion.

That's what I'd consider a good spec and a good LLM prompt.