"Cancer" is not a simple thing. Two people with "breast cancer" may have very similar or almost completely different diseases. As others here have pointed out, the magnitude and frequency of wins and losses matter even though trades are binary win/lose (which they can be because we've created an entire category of imaginary objects called dollars that can be counted).
Bayes rule is as applicable to any area of significant uncertainty, including win/loss magnitude. It is universal.
https://en.wikipedia.org/wiki/Binomial_options_pricing_model
seriously though, the example is just a model, and a simple one at that. it's certainly useful to call out where and how the model breaks down, but it's also useful to understand the conditions under which a model performs well. like most models, this one is not designed for black swan events (to which you allude).
also, the author acknowledges your point at the end: "...it is only part of a financial modeling solution" and "one needs ... to learn more through careful experimentation and study". you'd need to include some information about the magnitude of losses and gains, to be sure. i do like that he used a relatable example like sentiment analysis though.
Who said anything about that? Perhaps I was unclear, it's not about the ratio, it's about the streaks, you need to understand patterns of your wins and losses as it's critical to applying money management. If you only win 30% of the time, you could still make a killing if you can withstand the losing streaks without much drawdown or if the winners are big enough.