Do those guys even do high frequency trading?
Operating a dark pool and HFT are two different things.
Going to their web site and looking up the hype for their "Alph Pro", I see this:
3. Strategy Execution
Your instructions are executed using predictive switching to limit adverse selection costs by HFT. ...[hype]... As a result, your order is largely protected from adverse selection by high frequency traders.
So, the current scandal suggests that their algorithms didn't work so well, and perhaps they were deliberately ripping their customers off, but in any case I don't see any indication they were doing HFT themselves.