Although I don't think it even matters. They could easily cherry pick a period that is favorable for them. Wasting time and money on short term trading, rather than long term investment, using LLM or not, is never a good strategy for most people.
They backtested with public models, wouldn't the model weights already have the data? I double checked with chatgpt and looks like agents also had web search tool available so they could just lookup the past.
Although I don't think it even matters. They could easily cherry pick a period that is favorable for them. Wasting time and money on short term trading, rather than long term investment, using LLM or not, is never a good strategy for most people.