Earlier quoted context omitted.
Perhaps "quota" is the wrong term. Yes Google doesn't have a limit on how many people they can hire in total, but in the interest of diversity they are likely to _limit_ the number of "X" people they hire (where "X" is some over-represented ethnic group).
Uh, no, we're not. That is an entirely baseless assertion.
X is an over-represented group of Googlers threatening their diversity. Let's say it's males. Y is, let's say, women. Z is the total number of current Googlers.
In this case, 0.7Z = X and 0.3Z = Y. (70% men, 30% women).
Still with me?
If Google were to continue hiring X at the current rate (i.e., 70% of all candidates hired), then the diversity of Z will not change with respect to X and Y.
Therefore, in order to achieve the desired balance, Y must be >> X, be that through hiring (i.e., hire more Y than X) or firing (fire more X than Y). Rinse and repeat for other over- and under-represented groups at Google. I'm not sure why this concept is lost on you.