I think you give him too much cradit. Publishing a book means nothing because anyone can pay to have a book published -- but yes, that last bit is exactly the trait I was referring to as the main similarity amongst most crackpots. But the main point:
I don't buy the concept of a "serious autodidact" - I don't think I've ever heard a non-crackpot use it to describe himself. The word seems to me to just be a word crackpots use as an excuse/reason why they should be taken seriously despite not having a formal education in the subject they are trying to discuss. And Farsight's serious lack of knowledge of his favorite subject is clear evidence of the pitfall of the term. My theory is that most crackpots are people who stopped learning with the last thing they formally learned (or remember) from school and fill in the blanks with what they figure out on their own, which includes a lot of nonsense. So the main difference between TC and Farsight is the level at which they stopped learning. Yes, Farsight's level is much higher (high school or college freshman vs middle school for TC), but it is still a pretty remarkable failure to grow beyond that considering how basic/straightforward the concepts he butchers are.
Relativity is a good litmus test for self-learning because while the concepts are simple, they are also counter-intuitive. If a person can't learn a simple thing because it is counter-intuitive (so they refuse to believe it), that points to a serious learning deficiency.
I've (even as an adult) been told I'm arrogant about my intelligence, and that's something I try hard to avoid. As a kid, I would argue with my teachers a lot. Sometimes I'd be right, but not very often. In high school, I got in a long argument with my calculus teacher and he told me to just let it go and work the problem how he explained it and trust that when I got to the end I'd see that the answer and therefore method was right. And it was. That stuck with me. To me a "serious autodidact" is someone who is smart, but so arrogant that they reject learning from others and formal learning altogether and as a result, they don't learn much of anything.