People actually misstate the argument regularly. It is of course possible to beat the index over long periods, but there is little evidence that there is any skill that allows you to do it. That is, could those 3 individuals write down a prescriptive algorithm that allows someone to replicate their results ahead of time?
Another way to describe the problem is, given the same number of bots, randomly picking tades, as members of a trading forum, how many would beat the index? Is the number more than 3?
If there's no evidence that beating the market depends on any particular skill, that would seem to be saying it's essentially a matter of chance. If that is the case then it stands to reason that, as the period of time observed grows longer, it becomes less and less likely for anyone to beat the market.
I think you also have to define "the index". There are dozens of indexes on the NYSE alone, and many come with legitimate questions about how well they actually represent the market. In that sense, I think it is possible to choose (and not luck into) a better basket of stocks than what goes into the indexes. Of course, it isn't easy.
Another way to describe the problem is, given the same number of bots, randomly picking tades, as members of a trading forum, how many would beat the index? Is the number more than 3?