Clearly if "meaning is a category that human beings impose on an alien environment" then it is a product of their psychology which came into existence only when they did. Of course the cosmos does have structure but that isn't the same thing as meaning and Brown acknowledges that he isn't talking about anything that can be established scientifically.
To an evolutionist there is no need to try and make meaning or purpose retrospective any more than there is a need to look for a painting on an empty canvas. And one man's absurdity is another man's mystery. Chacun a son gout.
D W EVANS