Actually, I think they sometimes get mistaken for INTJs. The Ti-Si loop can create something of an fixation with trying to solve one particular problem. Andrew Wiles, the man who solved Fermat's last theorem, strikes me as one such person. He spent close to 10 years creating this one huge, proof. A work of genius, no doubt, but kind of a work of obsession at the same time. An INTJs projects tend to have stronger connections to the real world.