I spent a while looking at trading algos a few years back (partly because of quant stuff I got involved in, and partly out of curiosity). I found that none of the “slow” trading (i.e., that you could run at home alongside your day trading account) was substantially effective (at least in my sampling), but I never thought an LLM would be any good at it because all the analysis is quantitative, not qualitative or contextual.
In short, I don’t think this study proves anything unless they gave the LLMs additional context besides the pure trading data (Bloomberg terminals have news for a reason—there’s typically a lot more context in he market than individual stock values or history).