I think that the most likely explanation is that both environment and genetics are factors. In order to view inequality of outcome as proof of inequality of opportunity, you have to believe that group differences are due entirely to environmental conditions. That's a rather extreme position to take.
Of course genetics play a role - some people can get by on long term sleep of 4-5 hours a night, while most people need more. Some people have fantastic health from genetics (and then work hard to maintain it), while others are born with a slew of minor ailments that make them less productive. Not to speak of inteligence or natural talents, height etc.
