Idk man, this is pretty easy to demonstrate. Start with a trivial example: test is that input (2,2) -> 4. Function 1 does multiplication, function 2 does exponentiation. Both functions pass the test.
Sure, simple example but illustrative examples should be simple. But add more complexity and I'll add more examples of functions where the outputs are the same for a given set of inputs. (There's a whole area of mathematics dedicated to this!) It's simple, but you also confidently claimed something that was trivial to disprove.
Your claim is true if and only if your tests have complete coverage. So, your claim is only true if you've done formal verification of your code. Which was what I said in the beginning and is what Dijkstra claimed as well.
