First of all this isn't true. We cannot yet build something which has all the output associated with reasoning.

But secondary to that, symbols != semantics. If I had a magical machine which could appear from the outside to be reasoning, but upon closer introspection was simply simulating reasoning, by definition it would not be reasoning. And yet that is the criterion you seem to think indicates reasoning ability. So it's not a sufficient criterion.

Claiming we've figured out the fundamentals behind reasoning is hopelessly naive.