Freud feels a bit like Columbus to me. He set sail across the Atlantic driven by a very rich idea that was actually wrong, fired by emotions as much as concept, and reached somewhere that wasn't where he thought it was. Despite all that - or more likely because of it - he changed the world, though it was the people who came after him that sorted out what he'd really done, built on it and put it into solid theoretical and practical footings. That's really pretty typical of all human innovation - look at the electrical energy technology we have today, built on top of Faraday's explorations in the earlier 19th Century.
I haven't read much that Freud wrote himself, and my sense of him as a person is through Jung's autobiography. That's probably not the best place to find evidence of his type, given the fireworks between them! I don't know about ISTJ, but when he became established as an authority he certainly embodied a Parent / Superego archetype as far as I can see, and that tangled up with his ideas about the role of sex in human psychology. There's a lot of intuition in there to my mind as it needed a synthesis of existing knowledge and a leap of imagination to create a brand new and plausible theory out of it. But then it needed a thick skin, dogged determination, exercise of authority and an ability to inspire others and gain their support, loyalty even, to turn this into a success. It's hard for us to imagine now how unsavoury and disreputable this sort of concept would have been in the days when he first championed it. I can well imagine that INTJ would have been consistent with creating the theory, but ISTJ would have been consistent with his becoming the godfather of psychology in later life.