You missed the "for quarter time work". Earning $10k/month working 10 hours a week is pretty close to getting $400k/year for an FT job at Amazon (especially if FT ends up being more than 40 hours a week).
I say "pretty close" because if you're working on a 1099 basis there are a bunch of extra costs (extra SSDI payments, health care premiums, etc.) that you have to cover.
But still, nothing about the article suggests that the pay rates are poor, and the upper end ($250/hour) is quite good, especially when combined with the ability to work less than full time.