If you do everything badly (i.e. come up with bad business ides, cannot implement even those bad ideas, are bad at hiring, managing money, paying taxes etc.) then your success rate will be 0%, not 20%.
The issue here isn't about how hard starting a business is. It's obviously hard enough that many (most?) people who try will fail.
The issue is: can you do things better? Is doing things better lead to higher probability of success?
The "it's all luck" people say that you can't. There is no better or worse way. It's just luck. Therefore any advice is useless.
The opposite point of view is: yes. There are better and worse ways of doing things. And if you know what they are then you can tell other people. And that advice will help them to achieve success. But not guarantee.