My understanding is that HFTs - which are highly profitable - likely use some AI-techniques.
From my understanding of HFT, what they're effectively doing is automated front runnings.
They scan market activity and look for spreads between orders and availability. Then they place very short term orders any time they see, for instance, "I'll sell 1000 X at $49" and "I'll buy 500 X at $49.05", effectively buying up all the outstanding $49 orders and flipping them for a $.05 profit.
You don't need an advanced AI for this. You just need to be able to see orders and make trades faster than anyone else in the market.
Because getting out ahead of trade volume is so lucrative, you'll see huge investments in rack space near the physical stock exchanges and high speed lines between cities with big brokerages.
But AI trading is (theoretically) about spotting and predicting long term trends in the market, not front running active trades.