I did say 'practically guaranteed'. Let's say (for the sake of the argument) there's less than 5% probability there's something yet undiscovered about animal brains/nervous systems, that has qualitatively different computational properties than the current theory of computation says is possible in the physical world.
Does this sound excessively optimistic to you? I would be delighted to be proved wrong on this; it would be like the theory of relatively suddenly shook up Newtonian physics, in the computational domain.
But I'm not aware of any evidence that our models of computability have any holes. If you know of any, please let me know! I am a curious skeptic at heart :)