Proof for what? Smaller number of available women? That's obvious. That if larger companies hire the best of them, the remaining supply/quality will shrink? That's obvious too.
It sounds like they kept meetings deliberately off-the-record, but then again here on https://www.google.com/diversity/index.html there's stuff like "we’re committed to making our workforce more reflective of the world we live in". Number goals are unlikely to be published for obvious liability reasons.
Does anyone have any actual proof that Google is aiming for 50% or to represent the general population 1-1?