That's not necessarily true, it depends. For example risk parity is common knowledge but it still beats the market. You don't really need any secret sauce to use it effectively. You could do it, personally, and you would probably do well.

However if your strategies are well known people typically won't pay you much (if anything) to manage their money, because a bunch of shops will be offering comparable results with the same thing.

Continuing with risk parity: there are walkthroughs of how this works with code and math available online: https://cryptm.org/posts/2020/08/01/parity.html

Note the alpha, beta, volatility and Sharpe measures comparing a straightforward risk parity strategy to SPY.

It's not controversial to anyone in the actual industry that you can beat the market on a risk-adjusted basis. Very often the techniques for doing that are well known and can be levered up to safely beat SPY on a total basis with less overall risk. What's truly difficult (and secret) is beating the market by several standard deviations.