This is good background and all makes sense, but in the context of Melvin we don't have enough data to know whether any of this is true in their case vs just being part of their pitch. (This is essentially true of all hedge funds.)

What appears to be the case is they were edging the QQQ until they incinerated half their capital in a week on a bad bet and poor risk controls. So we now know they are uncorrelated with the S&P, but unfortunately we only know they are uncorrelated to the downside. (We don't know if their outperformance has been a result of real alpha or a strong market plus leverage.)

We saw a lot of these funds in the strong bull market of the 90s and the takeaway I remember was that it's impossible to know ahead of time who's going to blow up and lose all/most of your money.