If you buy green electricity such as wind power you are paying for that power to be generated. If 10% of people buy wind power, 10% of the grid should be wind. If 100% of the people buy wind power, 100% of the grid should be wind.
The parent is pointing out that absent the grid being 100% wind, the electricity delivered to your house isnt necessarily from wind power. This isn't particularly helpful, or even required. An electron is an electron, where it goes doesn't really matter, just where it came from.
I really think that should say, "If 10% of people buy wind power, at least 10% of the grid should be wind."
Alternatively, if folks suddenly demand wind and the supply is not there, I'd think there should be some sort of way for the companies to earmark money to meet the wind demand. This is especially true if the company is charging an optional premium fee for "green energy".
