They were successful when they were trading at lower frequencies and they hired people who implemented successful strategies that are pretty well known at this point to come work for them. People like Ed Thorpe had similar Sharpe ratios with less leverage so I think their returns are entirely realistic. It also took them about 10 years to get consistently profitable (under different names).

The low-latency networks you're alluding to exist but there is nothing nefarious about them. They aren't insider trading they are just getting market information faster than almost all other participants. I also don't think RenTech does this, it's more of an HFT strategy, and the HFT industry as a whole makes a few billion in profits per year, a lot of money sure, but it's not a huge amount when you consider how many transactions they are involved in.

Also, while the financial industry has caused some major blowups and bad actors often go unpunished it's a crucial part of any developed country and enables almost all big projects to get funded.