They are lying because you've never made money daytrading? As per my other comment, this is absurd. Being a profitable daytrader is very hard, and it's not probable, but definitely possible.
It's not a question of possibility, but a question of probability. Lots of things are possible with very tiny probabilities. That's why we do research, to estimate the probability of a phenomenon being true, in this case making money daytrading.
The research in the paper suggests that the probability that your friends are making more than a teller is 0.4%, therefore the probability that they are making less than a teller is 99.6%. So, if you have a hundred friends in daytrading, only one of them made more than minimum wage, two of them made money but at or below minimum wage, and ninety-seven of them lost money. Therefore, if they all told you they were making money, most (97%) of them were lying.
Look at the 10-k filings of companies. Lot's of people beat the market and hedge funds in particular. The thing is it's a hard skill to attain. Day trading is probably not the best, but the study should have focused on Full-Time trading. If you trade reversal/swing it's definitely possible to beat the market.
Just this year, I am at 35% returns, however, September is not in my calculation and this month was probably the most profitable so I should be around 50% at the end of the month if not higher (and this is a bad year.) Also the market has been choppy lately, the last few months so harder to lock in gains.
The research in the paper suggests that the probability that your friends are making more than a teller is 0.4%, therefore the probability that they are making less than a teller is 99.6%. So, if you have a hundred friends in daytrading, only one of them made more than minimum wage, two of them made money but at or below minimum wage, and ninety-seven of them lost money. Therefore, if they all told you they were making money, most (97%) of them were lying.