> If the gate only passes the digits 0-9 sent within 'x' seconds, and the code's job is to send a digit between 0 and 9, how is it non-deterministic?

If that’s all the code does, sure you could specify every observable behavior.

In reality though there are tens of thousands of “design decisions” that a programmer or LLM is gonna to make when translating a high level spec into code. Many of those decisions aren’t even things you’d care about, but users will notice the cumulative impact of them constantly flipping.

In a real world application where you have thousands of requirements and features interacting with each other, you can’t realistically specify enough of the observable behavior to keep it from turning into a sloshy mess of shifting jank without reviewing and understanding the actual spec, which is the code.