Aggressive behaviour is extremely risky, the payoffs and costs are both very high. And indeed, one of the potential costs is social ostracism. It counts under the taking risks part of 'taking risks and exploring'.
> why assume their apparent solitude is because they are “taking risks and exploring” instead of having nowhere else to go?
Pretty sure the point was to note that the "taking risks" part of that could have been the initial aggressive stance that got them ostracized. As the GP noted, there's a specific risk/payoff to behavior in a social group that might pay off well (more social status) or be very detrimental (ostracization), and given how detrimental being ostracized could be, I think many would consider that "risky behavior".