The argument is though that what Google is doing is hiring disproportionately from a smaller group of candidates. The argument would apply as well if they "looked for more fat candidates without lowering the bar" or "looked for more short people without lowering the bar". Those statements describe discrimination of people who don't belong to pointed group.

The argument in the memo is: hire on equal standards and if that results in less women than men then it's caused by X and here is what you can do about X instead of discriminating at the point of hire.