This is actually not all that true if you ask me. Based on my understanding, you need to know a shit load of stats in order to have a slight edge with blackjack, and not only that, but you also need to play not only your cards, but figure out what everyone else at the table is doing.
When it comes to trading, it can be just as simple as put on a trade, and it either hits my 2 point stop or 2 point target. After years of experience, you clearly develop rules for which trades you take, and which you skip, and this will increase win rates, but at the end of the day, the trade either works or it doesn't. I imagine what gets many traders in trouble, myself included, is micro managing trades. You end up getting out early cause you "think" it isn't going to work. Or you end up taking a smaller profit because you get "scared" it won't hit your target. Maybe there are some orderflow guys who can read the market very, very well and have a high degree of accuracy about where the next few ticks are going, but I haven't seen this.
Most people will not have a win rate that is over 75% for a typical R:R that is between 1:1 and 1:2. This means that we are all kind of still guessing. Its an educated guess, backed by lots of experience, or stats if you're truly disciplined and automated, but you still cannot in any way predict if this next trade will be a winner or not. But what you can do is ensure that it either hits your stop or target and then you move on. I have seen with my own trading that sticking to a certain R:R is much better over the long run than trying to manage the trade based on which way its ticking up or down. For every time I save myself a point from it not hitting my full stop, I have lost out just as many times on hitting full profit because I got out too soon.