One thing we need to be clear about is that you're not aiming to be better than average. You're aiming to make a profit. There are probably hundreds of thousands of day traders, there are probably <100 market makers and tradingfirms (far less than that for a some specific products) and you'll probably find 99% of the day traders aren't making systematic profits. There are lots of strategies that are much better than average and still worse than putting your cash in a bank.
You can aim for both. If you just aim for profit, then you can get lucky with just average, or even random, betting. If you find a weighted coinflip (which is not impossible), provided by how many times you can flip that coin, you will see steady systematic profits. Of course, majority of day traders are getting owned by the big players, and they would do better doing more reasoned and long-term investments. Most day traders are not even using predictive models though.
On that point - it's pretty clear that this Kaggle competition is highly likely to result in a decent number of submissions that make more money through luck, than other make through strategy.
