This seems to imply that the myth of Senna's greatness was created after his death, which I don't think is true. The myth was already there while he was alive. People worshipped Senna and many considered him the best driver in history of F1. A lot of this is due to his natural character as a good showman, high charisma and a sense of what people would like.
However, a part of that myth was true. He did have an incredible ability to turn a certain loss into a win: - Winning a race after his gears failed. Once, his car got stuck in the highest gear, couldn't switch to a lower one and still won - Consistently winning in Monaco, one of the most difficult racing track - Consistently winning in shitty, rainy conditions
I'd say it's more those things that created a legend out of him. Always being perceived as an underdog, and he often was, and still managing to win.