Despite being "rejuvenated" — de-evolved, even — into a "basic primate" by the massive overdose of the solution, Oliver is still able to say what he has said multiple times throughout the film, "I have nothing to say."
Even though Oliver was "rejuvenated" and "de-evolved" into a "basic primate" due to the massive overdose of the solution, he could still repeat his statement from the film, "I have nothing to say," multiple times.