So we're basically both right, but I was wrong in my initial assessment. Their crawler doesn't give them the page speed metrics, they get it from their toolbar data:

Page load time is the total time from the moment the user clicks on a link to your page until the time the entire page is loaded and displayed in a browser. It is collected directly from users who have installed the Google Toolbar and have enabled the optional PageRank feature.

http://www.google.com/support/webmasters/bin/answer.py?answe...