Normally, HFT firms don't care about insider trading. They are trading "noise" -- finding signals from past and present trading activity to make small bets that are 51% likely to be successful. They manage risk very carefully. This is not a secret. Cliff Asness from AQR Capital has given multiple long-form interviews on this topic. You can Google to find podcasts and printed articles.
On the other hand, long-short hedge funds, such as Steven Cohen's legendary Point72, love insider information! It is the backbone of their business model. The best ones employ an onion-like defense strategy to launder inside information as legitimate analyst reports. Only the outermost layer is directly transacting in insider information. As the information moves closer to the center -- real traders -- it is carefully masked to hide its origin. The SEC was never able to catch Steven Cohen, but they caught many, many people around him and eventually forced his original fund to close.
If a firm - that also has an agnostic active trading strategy - gets any inside info they have to cut the active trading strategy for liability reasons. That’s a conclusion some firms have made, not whether the SEC will actually bother them in a quantum reality or whether they can successfully defend it.
That said, RenTech was under intense scrutiny by SEC and IRS for their incredibly aggressive tax strategy. I did not follow it to the end.
First: Yes, Medallion fund is the primary fund of RenTech.
Second: To me, stat-arb can be HFT or non-HFT. They were/are doing HFT stat-arb in Medallion fund. AQR, Virtu, and Citadel are probably doing both HFT and non-HFT stat-arb. Twenty years ago stat-arb was called "pairs trading". Today, it is much more complex: basket vs basket.
They should have removed all orders the day they became an employee.
I know high frequency trading firms that are heavily inconvenienced by insider knowledge. They have to shut down so many things. Securities Insider trading is really tricky in the US as it isn't a specific law specifically about insider trading, only a bunch of court cases by the securities regulators that they sometimes won.