I think he means that even if you believe that power is zero sum (debatable) the system in which that power extends is not. E.g. technological advancement can shift power away from violent storms, earthquakes, viruses, etc and into humanities hands. Thus I think it's pretty clear that 'power among humanity' would not be zero sum

I kind of do. I guess zero sum is ill defined when it comes to power as it doesn't really have numbers attached you can add up.