Fibonacci (as implemented here) shouldn't be aided by any TCO. The execution trace will be something like:

  F(N) -- F(N-2) ...
   |
  F(N-1) -- F(N-3) ...
   |
  F(N-2) -- F(N-4) ...

But each has to return to the original caller because a summation still has to happen. The actual tail call is to the +/2 function.

F(N) can't complete until F(N-1) and F(N-2) have completed so it can sum up the values. If you pass the earlier computation, F(N-1), down to F(N-2) as a second parameter, you could get a tail call on the last part.

  fib(0, Val) -> Val + 1;
  fib(1, Val) -> Val + 1;
  fib(N, Val) ->
    T = fib(N-1,Val),
    fib(N-2,T).

(I think I wrote that correctly, can't test here.)

But that's a non-obvious tranformation. I really would like to see the compiler that recognized that this was a valid (computational) equivalent to the original.