[imtringued]

Yeah this is one of the weirdest takes ever.

"If you already know, terrific—but what are you meant to do if you don’t?"

You're supposed to look at the first gif that visualizes a waveform diagram. How are HDL designs tested? Testbenches (akin to unit tests) and model checking. With model checking you define the property you want to test and the model checker will try to find a counter example.

Said property is so obvious for fibonacci, that it is staring right at your face and you're consciously trying to avoid looking it in the eyes. Fibonacci is defined as fib(n) = fib(n-1) + fib(n-2), so that's what you need to test. This means you can simply test fib(1) = 1, fib(2) = 1, fib(3) = 2, for a fixed set of n to cover the edge cases, then choose a fixed set of random n and make sure that fib(n) = fib(n-1) + fib(n-2) is true. Obviously the only way to be 100% sure is to use a model checker and write code that is bounded in its runtime.