I was a little surprised by that as well, but my own analyses of Bitfinex data (together with the USDT/USD market on Kraken) brought me to a similar conclusion

I was a little surprised by that as well, but my own analyses of Bitfinex data (together with the USDT/USD market on Kraken) brought me to a similar conclusion that they are likely operating above board. That said, there have been previous studies challenging that conclusion so I wouldn't be surprised if they found more signals on a deeper inspection. The slide deck does go into the USDT/USD differences as well, and even mentions the large Bitfinex hack.