The point they're making is that fewer than 30% of monkeys would still be positive after so many years if it were random chance

Idk if that's true but you're not saying what percentage one would expect to see instead

I also think monkeys are the wrong example here because aren't they at even odds with the index? 50% of them, assuming they take no bananas for their service and assuming they don't get to make more trades than the index does, should have beaten the index, if my currently-half-awake brain is working correctly