But it does mean he isn't telling the truth, which is enough to make it grow. And besides, by that logic, lying at all is "just being wrong".
Lying isn't just being wrong. It's knowing the truth but knowingly reporting false information.
Being wrong, which could be what Pinocchio does in this situation, is simply not knowing the truth and then believing/reporting false information which you believe to be the truth.
Considering the fact that the magic which powers Pinocchio is capable of determining when he is lying and cause his otherwise inanimate wooden nose to grow, I think it's fair to expect the magic to be able to distinguish between Pinocchio actively telling a lie and just being an idiot.